PB 202 666
STATE OF THE ART: 1971 INSTRUMENTATION FOR
MEASUREMENT OF PARTICULATE EMISSIONS FROM
COMBUSTION SOURCES - VOLUME II: PARTICULATE
MASS - DETAIL REPORT
Gilmore J. Sem, et al
Thermo-Systems, Incorporated
St. Paul, Minnesota
April 1971
NATIONAL TECHNICAL INFORMATION SERVICE
Distributed .,. 'to foster, serve
and promote the nation's
economic development
and technological
advancement.'
U.S. DEPARTMENT OF COMMERCE
This document has been approved for public release and sale.
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STATE OF THE ART: 1971
INSTRUMENTATION FOR
MEASUREMENT OF PARTICULATE EMISSIONS
FROM COMBUSTION SOURCES
VOLUME II: PARTICULATE MASS - DETAIL REPORT
by
Gilmore J. Sen
John A. Borgos
John G. Olin
John P. Pilney
Benjamin Y. H. Liu
Nicholas Baraic
Kenneth T. Whitby
Frank D. Dornan
Theme-Systems Inc.
2500 North Cleveland Avenue
St. Paul, Minnesota 55113
This report was prepared under Contract No. CPA 70-23
with the Division of Process Control Engineering,
Air Pollution Control Office, Environmental Protection Agency,
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STATE OF THE ART: 1971
INSTRUMENTATION FOR
MEASUREMENT OF PARTICIPATE EMISSIONS
FROM COMBUSTION SOURCES
VOLUME II: PARTICULATE MASS - DETAIL REPORT
by
Gilmore J. Sen
John A. Borgos
John G. Olin
John P. Pilney
Benjamin Y. H. Liu
Nicholas Barslc
Kenneth T. Whltby
Frank D. Dorman
Thermo-Systerns Inc.
2500 North Cleveland Avenue
St. Paul, Minnesota 55113
This report was prepared under Contract No. CPA 70-23
with the Division of Process Control Engineering,
Air Pollution Control Office, Environmental Protection Agency.
THERMO.SYSTEMS INC.
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When U. S. Government drawings specifications, or other data
are used for any purpose other than a definitely related
Government procurement operation, the Government thereby
incurs no responsibility nor any obligation whatsoever, and
the fact that the Government may have formulated, furnished,
or in any way supplied the said drawings, specifications,
or other data, is not to be regarded by Implication or other—
wise, or in any manner licensing the holder or any other
person or corporation, or conveying any rights or permission
to manufacture, use, or sell any patented invention that may
in any way be related thereto.
References to named coimnercial products in this report are
not to be considered in any sense as an endorsement of the
product by the Government.
ThERMO- SYSTEMS iNC.
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TABLE OF CONTENTS
VOLUME I
FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . 5
ABSTRACT . . . . . . . . . . . . . . . . . . . , . . . . . 6
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 8
SUMMARY OF STACK EMISSIONS PROPERTIES AND INSTRUMENT
SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . 11
SUMMARY OF PARTICLE SENSING TECHNIQUES . . . . . . . . . . . 15
INDEX OF PARTICLE SENSING TECHNIQUES . . . . . . . . . . . . 17
REFERENCES . . . . . . . . . . . . . .. . . . . . . . . . . 87
VOLUME II
FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . 5
ABSTRACT . . . . . . . . . • , , • • , , , • • 6
SANPLINGPROBEDESIGN. .......... . •0•SSIS 7
DISCUSSION OF MEASUREMENT TECHNIQUES
MASS SENSING TECHNIQUES;
BETA RADIATION ATTENUATION . . . . . . . . . . . . 29
PIEZOELECTRIC MICROBAIIANCE . . . . . . . . . . . . 43
RESONANTFREQUENCY................ 57
GRAVIMETRIC WEIGHING , s . • p p p p • p p p p p 67
ELECTROSTATIC MEASUREMENT METHODS 73
OPTICAL SENSING TECHNIQUES:
LIGHT TRANSMISSION . . . . . . . . . . . . . . . . 98
MULTI—WAVELENGTH LIGHT TRANSMISSION . . . . . . . 115
LIGHT SCATTERING: POLARIZATION RATIO METHODS . . . 126
ANGULAR LIGHT SCATTERING . . . . . . . . . . . . . 134
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TABLE OP CONTENTS (continued)
SOILINGPOTENTIAL................. 145
OPTICAL COUNTERS AND PHOTOMETERS . . . . . . . 148
LIDAB . . . . . . . . . . . . . . . . . . . . . . 158
HOLOGRAPHY . . . . . . . . . . . . . . . . . . . . 163
MISCELLANEOUS SENSING TECHNIQUES:
ACOUSTICAL ATTENUATION AND DISPERSION .. . . . . . 172
ROT—WIREINEMONETRT................ 177
PRESSURE DROP IN NOZZLES . . . . . . . , . . . . 179
STACK EMISSIONS PROPERTIES AND INSTRUMENT SPECIFICATIONS . . 181
THERMO- SYSTEMS INC.
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FOREWORD
The compilation of the information contained in this publication was performed
pursuant to Contract 70—23 with the Air Pollution Control Office, Environmental
Protection Agency.
The information was compiled by Therino—Systema Inc., and their subcontractor,
North Star Research and Development, during the period 12 February 1970 to
April 1971.
Volume I of this report is written for the engineer or planner who needs to
know a few basic facts about a particulate mass measurement technique and wishes
to minimize, the time required to obtain this information. Volume I is intended
for use as a quick reference guide.
Volume II of this report is designed as a detailed in—depth report on operating
principles, techniques, historical data, and discussion of the more viable
techniques for particulate mass monitoring. Volume II is designed for the plant
engineer, abatement and control officials, and others who may not be familiar
with the detailed technology of these areas. Included are sections on power
plant emissions properties and extraction sampling probes.
Volume III of this report is a comprehensive survey of particle sizing techniques
which may be used by the plant engineer, abatement and control officials, and
others as a quick reference guide or as a source of more detailed information,
including references to original work 4
Volume IV of this report describes an experimental evaluation of the beta
radiation attenuation and piezoelectric microbalance techniques for mass con-
centration measurements on a coal—fired power generating plant. Problem areas
requiring further development are identified for personnel concerned with im-
proving the techniques.
This report is reviewed and approved.
F. C. Jaye
Project Officer
Environmental Protection Agency
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ABSTRACT AND CONCLUSION
Volumes I and II of this report discuss l known sensing techniques avail-
able for application to automatIc, continuous measurement of the rat ofpa tic—
ulate mass emissions from large fossil—fuel combustion facilities7 2
discusses techniques for automatic, continuous measurement of particle sIze
in the same environment. Volume I and II emphaèize particle
mass rather than other particle parameters . and all vo1un es emphasize emissions
downstream rather than upstream of any control ?“ Although the report•
emphasizes permanently—installed effluent monitoring systems, much of the infor-
mation is also applicable to portable and research instruments. •...
Volume I contains brief surveys of all known particle sensing techniques.
: A brief discussion of the principle of operation i followed by a list of
I inherent and practical strengths and weaknesses of each technique. A list of
I commercial manufacturers of related equipment and a list of references helps
the reader who needs more information on a specific technique. Recommendations
for further development outline areas of. needed improvement for techniques
which offer some promise for stack monitoring. The introduction includes general
comments which apply to all sensing techniques, and ranks all techniques in order
of present apparent potential. A separate chapter summarizes typical conditions
found in large fossil—fuel effluent gases and sets the necessary specifications
for a particulate monitoring instrument which operates in an effluent gas
atmosphere. .
Volume II (this volume) contains getail d is ns of particle sensing
techniques as applied to emissions monitorin . Eac’ d’iscussion analyzes pos tbIe
problems, and their solutions, in using the fechnique for emissions monitoring,
and includes an analysis of what particulate parameter the technique se ’es, how
closely the measurement correlates with particulate mass, inherent measurement
errors, practical design problems and possible solutions, the potential sensitivity
and response of each technique, the complexity of the potential instrument, the
present state of development of the technique, and recommendations for further
development. Each discussion includes a complete bibliography. A separate chapter
describes typical conditions found in large fossil—fuel effluent gases in greater
detail than found in Volume I. Anotherseparate chapter snm1n rizes many of the
problems encountered in the design of samDlinz Drobes reauired by most of the
particle sensing techniques. - • t
Accurate measurement of the particulate mass emissions rate requires an
instrument which directly senses the true mass of the particleè. Sensors that
are not sensitive to particulate mass, even though they may be calibrated against
a mass sensor, do not and cannot yield satisfactory correlation with particle
mass emissions during periods of changing and/or abnormal plant operating con-
ditions. Effluent characteristics, such as particle size and density, change
often in stacks. This may be a result of changes in combustion efficiency, fuel
composition, collector performance, or other system variables.
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All existing mass sensors require at least partial extraction of a represent-
ative effluent sample from the stack. No present commercial mass sensors meet
the requirements for accurate, long—term, continuous monitoring. Two techniques
could probably be developed into next—generation commercial stack monitors within
1 — 3 years: beta radiation attenuation and piezoelectric microbalance. Beta
radiation attenuation has been partially developed for stacks with several first—
generation commercial instr .nnents available. These instruments appear to need
design improvements. First—generation piezoelectric microba].ance instruments
exist for ambient air imonitoring, but no stack monitoring development has been
done.
Optical, or light, transmission is presently the most commonly used particulate
monitoring technique. It measures the optical density of stack effluents very
accurately If the instrument is carefully desigx ed. Unfortunately, few of the
presently available transmisaoineters are we1lud aigned f or accurate long—term
optical density measurements. Although light transmission offers several signi-
ficant instrument design advantages, the measurement does not correlate well
with particulate mass measurement, especially during changing effluent conditions.
Volume III contains discussions of automatic or semi—automatic particle size
measuring techniques as applied to emissions monitoring. The discussions emphasize
the particulate parameter (mass, number, surface area, etc.) which each technique
senses as well as the method of classifying particles into size ranges (aero-
dynamically, electrostatically, optically, etc.). Included are major features of
each technique, including practical problems which may be encountered when applying
the technique to effluent streams. Also included is a brief, but comprehensive,
survey of the many methods of expressing particle size, and an evaluation of which
are most useful for effluent particles.
Volume IV is the final report containing preliminary results of an experimental
evaluation of the beta radiation attenuation technique and the piezoelectric micro—
balance technique. Most of the experimental work was performed on an effluent
duct of a larger coal—fired power generating plant. The experimental sampling
system and the method of evaluation are described.
THERMO-SYSTEMS VNC.
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STACK EMISSIONS PROPERTIES AND INSTRUMENT SPECIFICATIONS
by: John Pilney
INTRODUCTION
The objective of this study is to characterize the effluent properties
and particulate emissions of large, stationary, fossil—fuel—fired power plants.*
The information presented in this report was sought through a comprehensive
survey of the published literature and other sources. This information is needed
to provide a guide for judging the merit of various techniques to continuously
monitor the mass concentration of particulate matter emitted by power plants.
It is also used to define the instrument specifications that a continuous stack
monitor should have.
The purpose of this section of the report is to characterize emissions from
coal— and oil—fired power plants only. Gas—fired plants were found to produce
so little particulate matter that they are not considered a source of particulate
pollution of any consequence. Discussed in the following sections are:
• The amount of particulate matter emitted by power plants
• Variables affecting the emissions
• Particulate loading
o Size—distribution of particulate emissions
• Control of particulate emissions
o Physical properties and the composition of emissions
• Characteristics and properties of the stack gases
Following this, a set of nominal effluent characteristics for both coal and oil-
fired power plants, and a set of instrument specifications, are presented.
TECIll IICAL BACKGROUND
The burning of fossil fuels generates pollutants in the form of aerosols and
gases, both organic and inorganic. The aerosols include all liquid and solid
particles including ash, carbon, and sulfur trioxide. The gases include oxides
of nitrogen, sulfur dioxide, hydrocarbons, and carbon monoxide. The particulate
emission resulting from the combustion of coal in power plants is formed from
*Large stationary power plants are those having at least: an electrical output
of 10 megawatts, a steam output of 8 x pounds/hr, or a heat input of
i 8 Btu/hr.
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coal ash and is referred to as fly ash. In oil combustion, however, the partic-
ulate emissions may contain a relatively large fraction of sulfates, mineral ash,
carbon, and carbonaceous matter in addition to ash. Emissions from oil combustion
are also commonly referred to as fly ash.
An estimate of the annual fuel consumption in a stationary sources in the
U.S. in 1965 365,407 show that equal heat equivalents of coal and fuel oil were
consumed. The estimated particulate emissions in thousands of tons per year from
stationary sources in the U.S. in 1965 was 7,000 for coal—firing (92 percent of
the total), 540 (7 percent) for oil—firing and 100 (1 percent) for natural gas—
firing. A stmvn ry of a recent estimate of emissions from coal—fired electric and
industrial power plants 727 is shown in Table 1. The figures shown for the
efficiency, amount,
These estimate are that
ef4±c—ienc.y zzof-O=perc-ent. In comparison, newly built, large coal—fired power
plants are being equipped ‘ 2,th electrostatic precipitators with an overall
efficiency of 98 percent 72 ’ and a few with precipitators having efficiencies of
greater than 99 percent. T iis information, including that in Table 1, illustrates
that in order of particulate pollution potential based upon mass, coal burning is
utmost, oil burning is second, and gas burning is third and of little importance.
The information in Table 1 also
Total particulate emissions from power production will probably continue to
increase in the future, if the present trend in population growth and the
standard of living continues. A look at the past, present, and future consumption
of coal and oil illustrates this assumption. For example in 1958, electric
utilities consumed approximately 156 million tons of coaF’ 26 ; in 1965 an
estimated 300 million tons 365 ; and, it has been estimated that by 1980, 440 million
tons will be consumed annuall 401 . Likewise, for fuel oil, the consumptions were
78 million barrels in 1958 52o and 1300 million barrels in 199 365; it is
estimated that 1900 million barrels will be consumed in 1980 9.
EMISSIONS FROM COAL FIRING
1. Variables Affecting Emissions From Coal—Fired Power Plants
The amount of fly ash emitted from coal—burning furnaces is related to
many factors. Some of the more important factors are gas velocity, particle size,
particle density, fuel—burning rate, combustion efficiency, furnace configuration,
and coal composition and size. An indication of how these variables affect the
emission rate is shown in Table 2 946. In addition, such-practices as soot
blowing and fly ash reinjection increase particulate emissions. In fly ash re—
injection, fly ash captured in control devices is injected back into the furnace.
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727
TABLE 1. Particulate Pollutants
Yearly Emission Eff’y.’ AmountC Extente Emissions
Production Factor of of of Tons/Yr
Tone Lb/Ton Control Control Control E
P e C C
_____ f c t
Source
1. Coal—Fired Boilers
a. Electric Utilities
1) Pulverized 265,200,000 l 6 Aa 190 0.83 0.96 0.80 5,049,000
2) Stoker 0
3) Cyclone 29,500,000 3A 36 0.90 0.99 089 58,000
Total from Electric Utilities 5,107,000
b. Industrial Power Generation
1) Pulverized 19,300,000 l 6 Ab 128 0.75 0.99 0.74 321,000
2) Stoker 68,600,000 14A 112 0.70 0.80 0.56 1,690,000
3) Cyclone 9,800,000 3A — 24 0.90 0.80 0.72 33,000
Total. from Industrial Power 2,044,000
Total from Coal—Fired Boilers 7,151,000
a Average percent ash in coal to electric utilities has been calculated to be 11.9%.
b Average percent ash in coal to industrial power generation has been assumed to be 8.0%, which is the
average of all coal produced in the U.S.
c Amount, of Control is defined as that fraction of the total production which has controls.
d Efficiency of Control is defined as the average fractional efficiency of the control equipment, prorated
on the basis of production capacity.
e Extent of Control is defined as the overall level of control, and is the product of amount of control
multiplied by the efficiency of control.
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TABLE 2. Some Variables Affecting Particulate
Emission Rates 946
Mass Emissions Rate
Variable Increasing Increasing Decreasing
Gas Velocity X
Particle Size X
Particle Density X
Coal Ash X
Coal Size x
Coal Fired in Suspension X
Coal—Burning Rate X
Coal Heat Value X
Combustion Efficiency X
Boiler Efficiency X
The majority of reinjected fly ash is slagged in the furnace, and is then removed
and hauled away. However, a certain portion of the reinjected fly ash escapes
with the stack gases. Thus, the amount of fly ash both challenging and penetrating
the control devices is larger when fly ash reinjection is practiced. In soot
blowing, steam jets are used to remove ash from heat transfer surfaces. Soot
blowing practices vary from plant to plant, and in some cases may occur as much
as 50% of the time.
Generalization concerning particulate emissions from the combustion of coal
is not appropriate because each coal burning power plant is strongly affected by
the type of furnace and amount and type of ash in the coal. Typically, coal used
in power plants contains 5 to 25 percent ash with an average value of around 8 to
15 percent. A rather comprehensive discussion of the properties, composition, and
distribution of coals can be found in reference 946 . A discussion of the dependence
of the type of furnace used on the characteristics of the coal is found in
reference 1 l 63 .
Basically there are three types of coal—firing furnaces: pulverized, cyclone,
and stoker fired. In pulverized firing, finely—divided coal is thrown or blown
into the furnace and combustion takes place In suspension. As the coal particles
burn and become smaller, many become suspended In the stack gases. In dry—bottom
‘ j rr .a IktF
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furnaces of this type, as much as 80 percent of the ash in the coal is emitted
from the furnace 8 S 7 ,1240; in wet—bottom furnaces, as much as 50 percent of the
ash is trapped in the furnace 1240 . The size and amount of the ash emitted from
pulverized—fired furnaces is primarily influenced by the fineness to which coal
is pulverized.
In stoker firing, coal is pushed or pulled into the furnace to form a burning
bed of coal. A whole family of methods of firing stoker furnaces are used.
These are referred to as the traveling—grate, vibrating—grate, single—retort
underfeed, multi—retort underfeed and the spreader stoker 0 Some 5 to 40 percent 857
of the ash from the coal is emitted from 8toker—fired furnaces with the flue gases
The araount of small particles in the coal, degree of agitation of the burning bed,
and the combustion rate are the major factors influencing emission rates from
stocker—fired furnaces.
In cyclone—fired furnaces, crushed coal is burned in a water—cooled, cylindri-
cally—shaped furnace. Because air is blown tangentially into the furnace, the
strong swirling motion of the air throws most of the coal to the wall of the
furnace, where the combustion rate is so high that the melting temperature of fly
ash is exceeded. This produces a molten layer of slagged ash on the furnace wall
that is continuously tapped off. From 80 to 90 percent of the ash in the coal is
trapped in the slag layer 857 1240 . However, ash that does escape from the furnace
is extremely fine. Because cyclone furnaces operate with a minimum of excess air,
the concentration of ash in stack gases can be as high as 2 1/2 pounds per 1000
pounds of gas 1240 .
More detailed discussions on methods of coal firing are found in references
946 and 1240.
2. Particulate Loading From Coal Combustion
Particulate emissions information found In the literature is reported in
several different units: grains per cubic foot of stack gas, grams per cubic meter
of stack gas, percentage of the ash in the coal, pounds of particulate per 1000
pounds of dry flue gas, and pounds per million Btu input. For the purpose of
this study, emissions in terms of concentration In the stack gas (either in
grains/f t or grams/m 3 )* are the most appropriate. Converting to units of con-
centration from one of the other units requires a knowledge of the fuel composition
and the amount of excess air used in firing the fuel. Frequently, the concentration
is corrected to some basis such as 12% CO 2 dry volume basis and 60 F and 30 inches
Hg.
*2.29 gm/rn 3 = 1 gr/ft 3
THERMO- SYSTEMS INC.
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Table 3946 presents uncontrolled particulate emission per ton of coal
burned for various types of coal firing units. Figure i946 is a nomogrqh
of the same information but with particulate emission expressed in lb/b 0 Btu
or lb/iD 3 lb flue gas at 502 excess air. Figure 2 shows the relationship between
TABLE 3 Particulate Emission Factors For Coal
Combustion Without Control Equipment 946
Particulate Per Ton
Type of Unit of Coal Burned,a lb
Pulverized
General 16A
Dry bottom 17A
Wet bottom without fly—ash ].3A
rein) ection
Wet bottom with fly—ash reinjectioub 24A
Cyclone 2A
Spreader stoker
Without fly—ash rein) ection 13A
With fly—ash rein) ectionb 20A
Al ]. other stokers 5A
Hand—fired equipment 20
aThe letter A on all, units other than hand—fired equipment indicates
that the percent ash in the coal should be multiplied by the value
given. Example: If the factor is 17 and the ash content is 10 per-
cent, the particulate emission before the control equipment would
be 10 times 17 or 170 pounds of particulate per ton of coal.
bvaiues should not be used as emission factors. Values represent the
loading reaching the control equipment always used on this type of
furnace.
type of fuel burned, excess air, and resulting volume of combustion products. Using
Figures 1 and 2, particulate emissions for coal combustion by six firing methods can
be estimated for 50 percent excess air and for a known heating value, ash content,
and type of coal. For example, using a typical heating value of 13,000 Btu/lb and
an ash content of 10 percent, Figure 1 gives the particulate emissions for all
stokers other than spreader stokers (case B) as 1.9 pounds particles per 106 Btu.
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PARTICULATE EHI $5 ION,
UPUEIICE lb!IO’Stu lb/IO ’lb flue qis at
LIRE 1 50% excess air
SEATING VALUE, ASH (Bituminous c a1)
1,000 Itu/Ib COIITNT, .2
20 30 0.3
18 0.4
0.5
20 0.6 0.5
15 IS A 0:8
14 I
13
— a a
II 7 2
3
to 6 c 43
5 54
9 4 F 65
8 108
2
A. CYCLONE UNITS
S. ALL STOKERS OTHER THAII SPREADER STOKERS
C. WET BOTTOM,PULVERIZED,OR SPREADER STOKERS
WITHOUT FLY—ASH REIIIJECTION
5 I 0. DRY BOTTOM PULVERIZED
E. SPREADER S1OKERS WITH FLY—ASH REINJECTION
F. WET BOTTOM PULVERIZ(D WITH FLY-ASH REINJECTION
Fig. 1. Nomograph for estimating particulate emissions from coal
combustion (without air pollution control equipment). 946
From Figure 2, the volume of dry flue gas per 1O 4 Btu fired is about 150 ft 3 for a
typical bituminous coal. Hence the estimated gas concentration of particles for
this case is about 0.9 gr/ft 3 . Similarily for the other firing methods (see notation
in Figure 1), the estimated loadings in gr/ft 3 are: 0.37 for A, 2.3 for C, 3.0 for D,
3.5 for E, and 4.2 for F.
Figures 3 and 4189 show the range and typical particulate emissions (gr/scf)
for pulverized— and cyclone—firing in relation to the percent ash in the coal. The
data contained in these figures agree quite well with those loadings estimated from
reference 946 .
Mass loading before and after electrostatic precipitators in three electrical
power stations operatin 3 under normal conditions and burning various coals are
presented in Table 4•331. Note that the loadings before the precipitator for cyclone
firing with fly ash reinjection are considerably greater than the value estimated
above for cyclone firing without fly ash reinjection.
ThERMO- SYSTEMS INC.
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UUNUR
C OIL
NATURAL
GAS
SUTANE
PROPANE
METhANE
‘ Volues corrected 60 °F and 30 in. lie dry
ANThRACITE
LOW VOLATILE
NI TUI4IIIOUS
) - 3USB TW4INOUS
) LIGNITE
)_IIO. 2 FUEL OIL
KEROSENE
GASOLINE
Fig. 2. Relationship between type of fuel burned excess air, and
resulting volume of combustion products. 46
I
0 20 30
PERCENTAGE OF ASH IN COAL
Fig. 3. Dust concentration from 189
pulverized coal firing.
2.0
,_ •s.
00
04
;:
0
Fig. 4. Dust concent tion from cyclone
coal firing.L0 9
10
I00
‘C
SO
TO
- 1
SEHIANTHRACITE
170
I i .
w
U’ NO
P C
w
50
150
R0
Mliii VOLATILE
30
20
hO
90
I
DUST CONCENTRATION PRON
PULVERIZED COAL FURNACE
0
10 —
I-
z
I I I
I)
z S
VU
0
I- ..
I
-J
I- 4
0
V
d 2
.
0
I
40
5 10 IS 20
PERCENTAGE OF ASH IN COAL
26
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TABLE 4. Particle Mass Loading Before and After Electrostatic Precipitator 334
Unit Particle Mass Loading (gr/ft 3 )
Capacity* Burning Rate Before Precipitator** After Precipitator** Overall Removal
( MW) Fuel ( Tons/Hr.) Unit 1 Unit 2 Unit 1 Unit 2 Efficiency. (Z )
223 Illinois 90 0.58 0.50 0.086 0.085 84.1
Coal &
Coke
223 Illinois 90 1.84 1.97 0.20 0.16 90.4
Coal
223 Montana 90 0.85 0.61 0.29 0.14 71.4
Coal
135.6 Illinois 60 2.16 2.54 0.069 0.074 96.9
Coal &
Coke
135.6 50% Illinois 60 0.88 1.03 0.049 0.043 95.2
& 50% Montana
Coal
135.6 Montana 60 0.78 0.90 0.063 0.11 89.7
Coal
580 Illinois 240 1.80_2.16*** l.57_1.82*** 0.017— O.0154_O.028*** 98.3—99.2
Coal 0.040***
*The 135.6 MW station is a pulverized—coal feed forced—air furnace; the 223MW and 580 MW stations
are crushed—coal feed cyclone furnaces. All three stations practice fly ash reinjection.
**The precipitator in each station consists of two parallel units; unit 1 and unit 2.
***At 29.92H fig and 32°F.
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Tables 5 — 8 present emissions information from two pulverized coal—firing
power plants. 415 Unit C is a corner—fired dry bottom unit and Unit D is a
horizontally—opposed—firing, wet—bottom unit designed for fly ash reinjection.
Particulate emissions from Unit C before fly ash collection are seen to vary
in accordance with the percent ash in the coal for constant load, Fly ash
reinjection in Unit D is seen to result in a very pronounced increase in the
loadings, up to a factor of about 3.7.
TABLE 5. Test Conditions and Major Pollutant Emissions — Unit C 415
Test
) o.
L 4
%
-
Soures
.
Coat
SulFur,b
%
.
Ash,’
%
Sampling
P iint
(Fig. 2)
.—
has
is—
Ply.Ash,
grs/bcf’
.- ppm by
N itrn en $ulfur Sulfur
Oxides’ Dioxide Trioxide
1
100
Ohio
2.2
15.1
B
A
3.7
0.19
500 1610
397 1470
381 1480
8
11
14
2
75
Ohio
2.2
16.8
B
A
3.5
0.08
304 1300
1250
13
7
3$
100
Ohio
2.0
17.0
B
A
4.1
0.24
430 1310
500
12
8
4
100
W.Va.
1.0
12.7
B
3.4
020
13
5
.
71
W.Vs.
1.1
11.2
.
A
B
A
0.25
2.4
0.15
404 7 00
346 600
473 370 1138 1Q78
5
10
8 12
£vern e
3.4
B: beriire Ily-ash rnllector.
A: eftcr fly-a’.h enlketor.
• Iiiw level bent ecnniimizer in operation.
lloL.ture and n.h-free basis.
• Moisture free l i—is.
4 Corrceicd to 12% CO,, dry baox.
• Iteported as NO 1 .
TABLE 6. Summary of Boiler and Flue Gas Data — U.iit C ’ 15
l tlrr C. suki ii,ii..
‘rC’s .
iO.
$Irnin
it . ..
tbjhr
(‘u i1
ll . Ii’,
tn/hr
Siiiiipling
l’nIpt
(1 ’iu . 2)
Vh.e—( ;i s
Vntui,w,
i.fun
Avt rngn
Vlsi..( ;
Tl’nqL, V.
Mniture, %
CO.’ %
0,, %
.
Enre Alr,b
%
I IkVi.IJIN) . .
•
2 0113,1W 42.7
‘
3 000, 1W 59
4 900,IXX) 54.2
5 700,000 31)
Aversute
11
A
13
A
13
A
13
A
B
A
32.501)
: 1 3 9 , 0 1 w )
2S.i,7 1K)
2.’ 16 ,5 III
11T,lI w )
392,urnu
373,1 110
3 5 3 , 1 K m
2$2,000
27 2, 0 ( K)
.
2S0
270
243
241
286
201
21 I
284
263
284
271 2G0
8.1)
9.
8.75
8.0
0.3
.0S
6.6
6.7
7.3
1185
7.4 7.8
14.4
14.4
15.2
14.8
14.1
14.2
14.7
14.3
14.6
14.6
14.6 14.5
4.7
4.6
3.8
4.2
5.0
4.9
4.5
4.9
4.5
4.5
4.5 4.6
211.3
27.4
21.8
24.4
30.7
29.9
20.8
29.9
20.8
20. l
26.8 27.7
B: belie 1ly—: —Ii c!k.tnr.
A: 11cr lIv.a .h r.illrrt.r.
• C.drtilde ’I rlie. Ia -it ui i.w’j cn valtio . and fuel analy c.
b Menanreit :tI I . i4t r’ ,1k”inr .: nverm o c cese air at boiler outlet was 10.3%.
Slantlard CuiuIit iit i—7U ’F and 29.0 sit Hg.
NOT REPRODUCIBLE
ThFRMO .SYSTEMS INC.
-------
—18—
TABLE 7. Test Conditions and Major Pollutant Emissions — Unit D 415
Sftfl%- .- E;nis ’sion
-Coal •. pling ppm by volume, dry b
Tcat •Lnad, Sulfur,’ Ash,’ Point FIy.Ash, Nitrogen Sulfur
Ko. % Source % % (Fig. 3) gra/sef’ Oxides’ Dioxide
-
a is
Sulfur
TrLoxlde
1 ’ 100 111. 2.3 8.8 B 7.4 347 1580
13
A 0.79 348 —
—
2 ilK) DL 2.6 8.1 B 2.0 429 —
—
A 0.50 331 —
—
3’ 161) IlL 2.8 8.1 B 6.1 453 1670
A 0.77 393 1500
12
“ 9
4 73 IlL 3.1, 7.9 B 3.7 421 1880
A 0.71 308 1840
6
6
6’ 73 III. 2.7 7.7 11 2.0 360 1680
7
A 0.51 347 1530
10
• lOt) IlL 2.1 7.0 B 4.0 341 1430
5
A ‘0.68 329 1260
—
Averagi’ 4.2 0.65 393 343 1051) 1530
9 8
B: before tlv—a.%II n.lk :cli,r.
A: after fly-n.h eulleetor.
• With llV—CVlI reiujCstioII.
‘Moisture atid n h free basis.
With f(y-a-h rcmjection and below normal excem air.
‘Moisture free ba .is.
‘Corrected to 12% COT, dry volume basis.
‘Expressed as nitrogen dioxide.
TABLE 8. Sununary of Boiler and Flue Gas Data — Unit D 415
1 ,iler Conditioni
Steam naI Sampling Fhso.Gn Average
Test Itate, Rate. Point Vohitno, Flue—(jsu
No. lIs /lir tons/hr (Fii . 3) , elin Temp., ‘F. Moisture, % CO , . % 0,, %
Excess Air’
1 140,001) 8.8 B 56,601) 310 0.5 12.7 6.7
A 00,301) 305 6.7 12.8 6.6
2 152,000 10.8 B 68,1KW) 312 6.1 13.8 5.7
A 0 5,6 ( X) 303 6.3 13.4 6.2
3 140,0011 0.1 B 00.500 311 5.8 11.7 7.0
A 65,1KW) 311) 8.5 11.8 7.1
4 105,000 6.4 B 43,61W) 318 6.7 12.5 6.3
A 44,001) 310 7.2 12.2 6.7
5 - 111,060 6.7 B 44,000 310 6.5 13.1 5.6
A 44,301) 301 6.0 12.5 6.3
6 147,000 9.5 B 57,501) 320 0.0 14.0 4.1
A 57,700 315 — 14.1 4.5
Average 315 31)9 6.R &S V’ I q 5 6.2
45.0 ——
45.1
30.8
41.4
48.6
40.R
41.7
45.7
35.2
41.7
23.7
2 6J 1
.1.tO 41.7
—
IS: bel”ri• lly— t—Ii r. ’Ik.u ’r.
A: s 1l s r llv.n.’t, ii ‘lk rIur.
?l1I:I%i,r,..I siP I lv-i. h rI,ll ,1•LHN: Av rngn r eo air at furn cu millet wos 37.4%.
Stsiis.Liruj (. ,,.,I.t,,,s., _.7 p ’J sitiit i.u , . Hg.
Table 9 presents particulate loadings before and after collectors for vertically
fired (Unit A) and a frQnt wall fired (Unit B), dry bottom, pulverized—coal unit .4l3
Note in Table 9 the high loadings for Unit A, the vertically—fired unit. The load-
ings of about 5 grains/scf for Unit A are about the upper limit of emissions from
pulverized—f ired furnaces.
Th ERMO- SYSTEMS INC.
-------
-19—
TABLE 9. Fly—Mb Collection Efficiency 413
Unit A
fly -Au. Ln&ding.
grrainis/erP
Inlet Otitlet
B-
FLv.A rh Fly-Ash Lnnding.
Collection grain/ert
Efiicicncy, ‘4 Inict Outlet. -
Fly -Mh
CoUe.t i’n
Eflicieiicv, ‘
Avg.
4.8 0.15
5.0 0 IS
4.7 0.19
4.S 0.18
-
UnitA
Fly-Ash Loading,
—grain .u/sef
Inlet Outlet__-
96 8 2.7 0.7 i
96 4 2.8 0.37
96.0 2.3 0.21
96 4 2.5 0.44
Partial-Load Teeta- -
tnitB
Fly-Ash Fly-Ash Lo id ng,
Coaectu n .—irr.iine/5cf—’
Eiflcieniy, ¶C Inlet Outlet.
73.9
$5.7
3 I
-i
.
Fir-Ash
Collect inn
EWicienI3,
Avg.
4.2 0.09
5.2 0.14
4.7 0.11
97.7 2.9 0.33
073 1.9 0.12
97 3 2.4 0.22
$5.7
941)
91.3
Corrected to 12% CO. Dry
Volume Bnuis
—
NOT REPRODUCIBLE
Compared in Table are findings from the units described above from
references 413 and 415 with those from a cyclone fired wet-bottom unit and a
traveling—grate spreader—stoker. Concentrations under partial and full—load
tests of each unit are presented. The concentrations all seem to decrease
with a decrease in load except for cyclone firing. The concentration from a
cyclone furnace could, however, be expected to increase with decreased load
because of a corresponding decrease in the centrifugal—force effect on the
particles in the furnace. The data also shows that pulverized—fired units
produce higher particulate concentrations than cyclone or spreader—stoker firing,
even though the spreader stoker and the cyclone utilized fly ash reinjection
and the pulverized units did not.
Table 11 shows the particle loadings before and after mechanical collectors
for a spreader stoker (Boiler No. 1) and a pulverized—coal stoker (Boiler No. 2),
all without fly ash reinjection. 744 These data shows that using pulverized coal
in a stoker increases emissions
Tables 12 and 13368 suimn rize the design and operational conditions, and
emissions from coal, oil, and pa hea generation sources. For the coal units
listed, the emissions in gr/ft 3 at 60 F can be estimated by multiplying the
values in pounds per 1000 pounds of gas by 0.6. For example, the uncontrolled
emissions from the chain—grate stoker are, therefore, estimated to be about 0.6
gr/scf for 50 percent excess air. This value compares well with the value
estimated from Figure i9 ’ 6 for a heating value of 13,000 Btu/lb and ash content
of 7 percent.
ThERMO- SYSTEMS INC.
-------
TABLE 10. Fly—Ash Concentrations and Collection Efficiencies 4t4
Type of Boiler
- Firing
Mhin
Coa 1
%
Concentrations
Type of
Fly-ash
oiiectord
Collector
Efft iancy
r/ecfb
S
iba/1 lbC
dry flue gas
A
B
A
C
C
,
,
C
Là
1
0
.J
, ,
C
4
I.
QI
Vertical (Pulverized)
20.2
.8
o.t8
——
8.8
0.27
C, I
96.1 1
Corner (Pulverized)
l1e.9
3.7
0.23
6.9
0. 42
C, 2
-
93.9
Front-walt (Pulverized)
1.0.3
2.5
0.lele
li.6
0.82
—
2
83.1
Spreader-stoker
—
8. le
2.3
0.38
—___
.2
—
0.66
C
83.9
Cyclone
7.7
1.5
0.39
2.8
062
2 -
7’;.5
iorizont 1 o sed
- (Pu
Vertical (Pulverized)
8.2
19.0
ls.9
Ii. 7
0.68
0.11.
8.9
8.7
1.27
0.21
C
C, 2
- 83.9 -
97.5
Corner (Pulverized)
-
135
—
2.9
——-
2. 4
0.13
—
5.5
0.21.
C, I
95.T
Front-vail (Pulverized)
. 9.2
0.22
11.Ii
0.1(1
.
2
-
91.3
Spreader-stoker
—
8.7
1.5
0.1.9
2.8
0.35
C
.
87.3
Cyclone
7.11
1.8
0.22
3.1
0.36
—
. 2
86.3
Horizontally opnosed
(Pu lyerjzecfl
7.8
2.9
0.61
5.1
1.1
-
C
77.7
5 Mo isture -free basis.
bCorrected to 12 percent CO 2 dry vohmie basie.
pounds of dry flue gas corrected to 50 percent excess air.
designates cyclone; £ designates electrostatic precipitator.
°Average values for either three or four tests at each unit.
Av.rage values for two test. at each unit.
B: Before fly-ash collector. A: After fly-ash collector
-------
—21—
TABLK 11. Flow and Grain Loading Results 744
Grain Loading
Boiler I ACFM ScFM* Temp.°F ( gr/SCF )
i - Xn let 167,000 13.1,000 340 0.91
Outlet 176,000 117,000 340 **
2 ‘Xnlet-
East 110,000 73,000 340 1.99
Outlet-
East 132,000 87,500 340 1.67
Inlet-
West 305,000 69,500 340 1.99
Outlet-
West 115,000 76,100 340 1.3].
* Standard conditions 70°F, 30°Hg.
** Boiler leaking, could not test
Variation of particle concentration across a duct is a factor that can
strongly affect the reliability of emission rates estimated from manual sampling
information, One investigation 590 experimentally determined the distribution
of particle loading in a vertical duct. The resu].ts are presented in Figure 5.
These results show the loading lowest in the center o the duct and highest near
the four corners. However, because a cyclone was used for sampling, the data may
be baised towards the larger particles. Unfortuna ely, no mention is made of
the size and type of firing equipment, and no velocity distribution is given.
‘-‘ .,.I .-Lá% %, Ir&IC I If’
-------
TABLE 12. Design and Operational Sinmnnry——Heat Generation Sources 368
uii*
No.
—
Fuel
1. sed
Firing Method
Design Data—
Type of Unit Utilization
Rated Cap uity
per hr
10 Lb Million
Steam Btu
Dust
Collector
——Fuel Data—--
as-iecewed basis
Vola-
tile, Ash, ,
‘
—
Fuel Rate
—
Lb.
Operating Co
Gross Btu
Input
Per Hr
Milhon Sm
ndiiions
tesm
Rate
— .
I0 Lb
During Test
-team
Pre uire
Psi
—
Smoke,
Opacity,
1
2
Coal
Pulverized dry
bottom
furnaces)
Water-tube
boiler
Electric
generation
Process
heating
1060
200
Mechanical
electrical
Multiple
cyclone
31 20.2 2.3
36 4.3 0.0
132.000
9,430
1560
130
1121)
106
21 )0
307
3040
60
3
4
Chain grate
stoker
8preader stoker
(with reinjec-
tor)
Water-tube
boiler
Electric.
generation
Process
beating
125
70.5
None
Multiple
cyclone
•
46 70 3.8
37 4 7 0.8
62,400
1,290
647
59.2
111
49
430
16 1)
20-40
020
5
6
7
rnderfeed
stolen
Fire-tube
boiler
cast-iron arc-
tional boiler
Process
heating
School
beating
Home
bcnlrng
7.2
3.8
0.26
None
36 4.7 0.7
19 5 0.8
38 3.9 1.0
317
214
4.8
4.4
3.0
0 066
.
110
37
9
20-40
0-20
020
8
Hand-stoked
Hot-air
furnace
Home
beating
0.20
38 2.7 0.5
8
0.115
.
10-80
9
10
Oil
Steam-atomized
Water-tube
boiler
-- -.
Process
beating
23 23
30 30
.
None
No.2FuelOil 3.2
(28.30 API)
No.BFuelOil 0.7
(13.5° API)
1.110
769
21
14.4
17.9
10.3
230
123
5
5
11
Low-pressure
sirustomized
Scotch-marine
boiler
Hospital
heating
4.2
No. 1 Fuel Oil
(43.5° API)
35
0.70
93
0
12
13
14
Centrifugal-
atomized
Vaporized
Cast-iron see-
lionel boiler
Hot-airfurnace
.
Hot-air
furnace
Home
heating
Home
heating
025
0.14
0.09
.
No. 2 Fuel Oil
(31.5° API)
No.2FuelOil
-- —— (31.5° API)
No. 1 Fuel Oil 0.05
(13° API)
88
4.4
1.2
0.17
0.085
0.025
0
0
0
15
16
Gas
Premix burners
Fire-tube
boiler
Scotch-marine
boiler
Process
heating
Hospital
heating
7.2
4.2
None
.
Natural gas (94.2
methane 3. 6
ethane)
402
42
9.3
0.98
109
98
0
0-21)
17
18
19
Double-shell
boiler
Hotairfurnaee
Wall apace heater
Rome
heating
0.18
0.21
0.02.5
7.9
-- - -
7.4
0.32
0.18
0.17
0 012
0
0
(1
‘Gross
heat input.
-------
TABLE 13. Pollutant Emission Sinmn ry — Heat Generation Sources 368
1”hie (ies Condition, in Stack
—I)rj Basis—
Source Fuel l’ir ,ne Flow, Tonip, 11,0, CO O
N’. lJec ’l Mvtl,n.! ant ” ‘ I % % %
I Coal Pi ,R , ’ri,cd 411,000 200 1 4 12 3 6 9
2 32,300 233 4 1 2 12 1 6 1
3 C!,n’n Crete 43,1)00 •l ’iO 1, 7 12 1 7 7
stoker
4 S r.’n.Ipr 18,100 403 41 9 10 0 3 3
aoker
tn,l,.rfred 1.340 ‘1110 2 I ‘1 0 17 2
6 itoka’, 3 ,2’#C ) 2 ’ 2 1 2 1 111 1
7 41 ‘141 22 26 171
S 78 220 2.1 28 177
hand-
atokod
145 170 3.0 2.9 16 9
113 173 2 6 1 8 18 3
.19 183 I’l 12 1143
3.1110 380 84 36 143
323 210 11.4 3 ‘ I 100
“2 171) 4 11 2.1 10 1’
2 141) 4 I 2 2 I? ‘1
II 2111 II 2 (4 173
—-———Total Particulatea —
O ,d esoI
Ben- Csttmfl hydrocarbons ,itrogen .—Oxideq of Sulfur—
anne- Monoelde (as Methane) (as %Op1 (as S0 7 )
Lb Per Soli,— Lb l’er Lb I’er Lb Per .—Lb Per—.
Ton ble Ton Ton Ton - Ppm Ton
1000 1il1ion of Or- Million of Million of MiU ,oa of by Million of
Lbb Btu fuel’ gan’ca Btu fuel’ Blu fuel’ Iltu fuel Vol. Btu fuel’
030 039 140 07 0001 (11 0007 010 047 11 1490 372 68
1 90 2 ‘23 61 6 0 3 0 10 2 8 0 004 0 11 403 1 00 ‘ 28
0 99 1 ‘31 31.0 0 3 0 311 12 0 003 0 II . 2030 41 14 146
066 082 22.6 14 <01 <3 0000 016
O 8 062 170 11 016 431 0116 32
024 023 71) 36 014 39 0031, 10 030 83 203 23
0 32 0.44 12 1 2 1 I 31 0.12 3 3 0.36 9.8 178 ‘. 2
1.80 1.20 37 17 3 31 99 0 73 21 0.11 3 2 80 0 314
<0 1 <4 0 013 0.31 1230 3.0
NOT REPRODUCIBLE
p..,
I ’ .,
p
11
10
11
1 1
5,200 3110
10,000 340
193 231)
83
‘14
7.4
oil Steam-
— tOYPlited
Loa.
press ure
air—
atoti,izt’d
Centrifuruil-
uit ,i ut,i ,et l
Vanorienul
(“as Pra’niix
burnpr
0.0 82 032 0306
0 267
8.8 ‘1 0 0 049 0.011
12
13
14
13
16
19
—For,ua!akd.yde-’-— ’
—Lb Pt’,
Ton
Million of
Utu fuel’
1.3 X 10’ 30 X 10’
0.9 x 10-’ 241 X 10’
1.4 X 10’ 33 X 10 ’
2.2 X 10’ 60 X 10’
2) X 10 4 3190 )C 10”
62 38X10 ’ W0X10-
52
11
116 0.63 X 10’ 24 X 10’
117 10
10 0 2.7 0 0331 2 2 0.001 0 17
20 60
0.31 12 188 I 3 48 2 4 X 10-’ 8 X 10
1231 0 33 14 1.6 X 10’ 62 X L0 ’
• Dinah. in the tahIt’ indicate that ni. test waq i,.ade.
“ ‘nunuk of p .rtit’iiI’itr per 10(11) imuinik’ i.f dry Out’ g’is ui4jutta”l to 310% e ces. air,
Al’! gra itiee of the foal ouls tire given in Tahule 1: the ,lensitv of natural get — 0.0143 lb per cu ft C601 I atm)
0 041 0 040 1 8 39 0 038 1.8 0 44 17
0070 0.080 31 94 0075 29 0021 082
o 067 0 07! 2 S 11 0 23 V 8 0.030 1 2 0 03 1 3
0 026 0.021 1 0 11 0.013 0 6 0 003 0,14 0 14 6.4
0 o:to 0 0:12 1 1 3 0 1 00 140 0 082 1 8 1) 16 7 ‘1
0 010 1) (1131 0 ‘4 33 0 02 0 9 0 3:. 16
1 oi I 0 007 a :t 2’I 0 0241 1 2 0 1)22 1 0 () 09 4 i
(1 027 0 026 1.2 114 0 010 l_4 I) 0)6 0 74 0 041 2 8
14 012 4.3
311 0 46 18 &4 x 10’ 2310 X 10 ’
4 0.08 3 3.8 X 104 230 X 10 ’
0 8 ” X 10 ’ -11 X 10-’
2 2 X 10 ’ 100 10-’
(I (1 0 2 4 X 10-’ 110 X l0 ’
I) 0 0 1 1 X 10’ x 10W’
2G X 10 ’ 19111) X 10 ’
-------
—‘. .,—
4
Fig. 5. Dust concentration patterns at constant
load (gr/ft 3 ). 590
3. Particle Size of Emissions From Coal Combustion
Particle size of fly ash from coal combustion ranges from less than 0.1
micron to several hundred microns. 1 ° 19 For most uncontrolled fly ashes, over
90 percent of the particles by weight are In the range of 1 to 100 microns, with
the mass median diameter commonly in the range of 20 to 50 mIcrons. Particle
size results obtained independently by different laboratories on samples of the
same ash may show substantial differences.’° 1 - 9 Particle—size distribution for
many fly ash samples closely follow the log—probability law. It should be noted
that most particle size data was obtained by the Bahco sizing technique, which
has a lower size cutoff of about 1 micron.
Typical sizes and ranges of size distribution of particulate emissions
before collectors are shown in Figures 6 — 9 for pulverized—, cyclone—, spreader—
stoker—, and stoker—fired furnaces. 946 The data from which these curves were
compiled were obtained in the literature.
THERMO- SYSTEMS iNC.
-------
—25—
I
e
I
Fig. 6. Estimated Size Distribution for Particles from Pulverized—
Coal—Fired Furances (Before Collectors). 946
Fig. 7. Estimated Size Distribution for Particles Emitted from
Cyclone Furnaces (Before Collectors). 946
MU N N N uu
N1NIT I? IIiN T its* mi* sTuu ,&u,sc,t sl$
I
I
TN
II
SN LNSIC2 SI S I I II N N SI N N 75 IS N I N N NINS NI
•II’TST II III II IL I ISMS 11 1 ( 17 “ShILl 1171
ThERMO- SYSTEMS INC.
-------
-26—
1
a
Fig. 8. EstImated Size Distribution for Particles Emitted f 9 m
Spreader—Stoker—Fired Furnaces (Before Collectors). ‘ 6
Fig. 9. Estimated Size Distribution for Particles Emitted from
Stoker—Fired Other than Spreaders) Furnace (B f ore
Collectors) 9’ 6
II 101113 II I I I N U U N N N U N N U U Uuis i.o
Kk(UI IT IIIITT Lili III ITITU PiI1 I SIN
I
‘U
IS I NIJII II 3 3 1 II U U S N U N N N U N U NINS II
NINE S I hINT dIT Tifi ff1111 PIEINI sal
ThERMO-SYSTEMS INC.
-------
—27—
Figures 10 — 12 show particle size distributions of i sions for
pulverized—fired, cyclone—fired, and stoker—fired The size
distributions were all determined by the ASME PTC 28 method of analysis (Bahco . 9
The dashed curves bracket the size range and the solid curves indicate the most
probable size distribution. These size distributions and their ranges do agree
fairly well with those in Figures 6 — 9.
70
50
40
30
‘0.
10
7
I
4
5
2
I I S lOGO 40 40 109095 99
PERCENTAGE ST WEIGHT LESS THAN SIZE
9,.,
Fig. 10. Particle size distribution for
coal—fired furnaces. 189
l00
To
So
40
30
10
W I
to —
pulverizer—
I I S 20 4050 509090 99 99.9
PERCENTAGE ST WEIGHT LESS THAN SIZE
Fig. 11. Particle size distribution for cyclone—coal—
fired furnaces.’ 89
ThERMO-SYSTEMS INC.
:1 I 11111111111 I 1
90% OF :
•
• AVERAGE SPECIFIC —
GRAVITY 2.34
• U
• .
• U
•
•
•
—
a
S
I• I 1 I 111111 I I I I I
S
S
a
•
S
U
U
•
a
S
a
a
II I I.
‘U
,/ :
AVERAGE SPECIFIC :
SR ITY 2.79
I
II,
I, I U
U
I J I
7 ,1/ -
S
1
I
1’ 90% OF DATA —
I I 111111 I II I I
-------
—28—
70
$0
..4 0
30
20
110
57
:
0.1 I 5 10 40 90 $0 90 95 99 99.9
PERCcNTAGC BY WEIONT LESS THAN SIZE
Fig. 12. Partie].e size distribution for stoker—coal—
fired furnaces. 189
Figures 13 and 14 show size distribution of emissions far pulverized—fired
furnaces and stoker—fired furnaces other than spreader stokers. The data
reported in these figures were recently obtained from several power plants.
For comparison, the corresponding range of size distributions and typical size
distribution reported in reference 946 are included and given by the dashed
lines. Even though the size analysis methods used were nat all the same, there
is reasonably good agreement among the data. Also, although most of the curves
do lie within the corresponding range reported in reference 946, the most recent
data gathered during this study indicates a narrower size range and smaller
particles.
Number—size distributions of fly ash before and after electrostatic pre—
cipitators for a cyclone—fired furnace and a pulverized—fired furnace are shown
in Figures 15 and These curves show that the size distribution of fly ash
after a precipitator is smaller than it is before a precipitator. The electro-
static precipitators were the only control devices on the furnaces, and a portion
of the precipitators were being cleaned by rapping when the samples were being
collected. The sizing was done with an optical microscope on a filter sample.
AVERAGE SP2CIFIC
GRAVITY 2.32
S
S
S
I I I I III I I I I
a
a
a
a
a
a
a
__________________ •i I
a
I I I I I 11111 I II
ThERMO . SYSTEMS INC.
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—29—
S
C
C)
‘P
2 10 30 50 70 90 98
Percent by Mass Less Than Stated Size
Fig. 13. Particle Size Distributions for Pulverized
Coal—Fired Furnaces.
:LO0
S.
C
0
I a
U
S
•-— 10
a
C )
N
“ -I
V I )
(3
S i
A
A.
2 1) 30 5070 ;o 98
Percent by Mass Less Than Stated Size
Fig. 14. Particle Size Distributions for Stoker Coal—
Fired Furnaces.
100.
1
ThERMO SYSTEMS gNC.
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—30—
w
N
.,-I
U)
14
99.5
99
98
95
90
80
60
40
20
10
5
2.
1
0.5
0.2
0.1
0.1
Fig. 15.
99.5
99
98
95
90
80
60
40
20
10
5
2
1
0.5
0.2
0.1
Fig. 16. Particle Number—Size Distribution for
Fired Furnace. 334
Particle Number—Size
Fired Furnace. 334
Distribution for Cyclone—
Pulverized—
TTTLL
N
“4
U)
— I I $
/
After
— Electrostatic / _/
PreciPitator
//efore
a
Electrostatic
/ ,‘• Precipitator
— I
/.
—
—
I I I I I liii
—
a
0•
I I iiiiTt
1.0 10.0
Particle Size, (microns)
ParticW’Size (microns)
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4. Control of Particulate Emissions From Coal—Fired Power Plants
The amount and efficiency of control of particulate emissions from coal—
fired power plants are shown in Table 1.727 This table shows the extent of
control to be higher for electric utilities than industrial power generation,
although the emissions from utilities are still about 70 percent of the total.
It also shows that most coal—burning power plants have some type of control
equipment.
The control equipment used is the settling chamber effects of large
breeches and chimney bases, mechanical collectors such as cyclones electrical
precipitators, or combination mechanical—electrical precipitators.’ 88 Efficiency
ranges generally achieved by these collectors on various units are given in
Table 14.946 Thus, if the uncontrolled emissions are known or can be estimated,
the controLled emissions of particles can be estimated from a knowledge of the
type of controls.
Table 14 USUAL EXPECTED EFFICIENCY RANCES FOR
COMMONLY USED CONTROL EQU MENT (percent) 946
Type of fITIn
— or fiarnscs
Type .1 control .qu;pment
tiectrostatic
prectpltalor
XI h.
efficiency
cyclone
Low.
reSiotanci
cyclone
Settling chamber,
c*puided
chimney basee
Cycloute
Pu!verI,.
Spread.r stoker
Other utokers
aS.9?
80-99.?
•
•
30.40
63.?
84.90
90.95
40.30
40.80
70.40
75.85
•
.
80.30
83.50
•Thc P.igher efftrlencae , can only be atta ned with high.elftcicncy cyclones in
Sen., w,ih . sctro.tatlc prec p%tsIon..
NOT REPRODUCIBLE
5. Physical Properties and Composition of Particulate Emissions From
Coal—Fired Power Plants
In general, the physical properties and chemical composition of particulate
emissions vary considerably, probably because of the large number of variables
affecting them. This undoubtedly is the reason why no information was found in
the literature relating the physical properties and chemical composition of the
emissions to the coal composition.
a. Particle Density. Density of particulate emissions depends primarily on
particle size, structure and composition. In general, the relatively large, coarse,
lace—like particles containing a high percentage of carbon have a low density of
the order of 0.6 to 1.0 gin/cm . The finer particles of ash, which tend to be low
ThE MO-SYSTEMS INC.
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in carbon, have a much higher density, usually in the ran e of 1.5 to 3.0 gm/cm 3 . 1019
The average density of fly ash is 2.0 to 2.7 gm/ca3. 946 4 ’ 2 The density variation
among constituents of fly ash is quite large; the magnetite—hematite particles
have a value of about 5 gm/cm 3 , hollow glass spheres of less than one, and dense
glass spheres between 2.0 and 3.0 gm/cm3. 442
The average particle density of fly ash can therefore be expected to decrease
as the carbon content of the ash increases. A decrease in combustion or boiler
efficiency, or an increase in the burning rate or gas velocity will most likely
result in an increase in the carbon content of the ash. An increase in the silica
content of the fuel could either increase or decrease the average particle density
of fly depending upon whether a predominance of hollow or solid glass spheres
results from firing the coal.
b. Bulk Density. Dry, uncompacted fly ash weighs about 55 pound per cubic
foot and, when completely compacted, 70 to 80 pounds per cubic foot) 2 The
bulk density can be expected to vary in accordance with the particle density.
c Particle Shape. Glass, the abundant constituent of fly ash, is present
in the shape of spheres, broken hollow spheres, ellipsoids, teardrop, and
irregularly shaped particles. Magnetite—hematite particles, the second most
abundant constituent, is present in the shape of spheres. Carbon particles vary
widely in shape, but most of them are present as highly irregularly—shaped 442
cellular particles that increase in abundance with increasing parti } size.
The size of carbon particles generally range from 10 im to 300 inn. 1 ”
d. Electrical Properties. Much information exists on the electrical
resistivit of fly aah. 422 ’ 434 , 496 , 608 , 744 , 1 -°’ 9 Resistivity normally varies from
108 to 101 ohm—cm. Electrical conductivity of fly ash particles is increased by
absorbed SO 3 and by increasing carbon content. Resistivity of fly ash greater
than 2 x i0 10 ohm—cm hampers collection in electrostatic precipitators.
e. Chemical Composition. The major constituents of most fly ashes are
silica 1 alumina and iron oxide. 323 ’ 44 ”° 19 Typical coal ash analysis limits
are:lOi. 9
Constituent Percent by Weight
Silica 30—60
Aluminum oxide 10—40
Ferric oxide 5—30
Calcium oxide 5—20
Magnesium oxide 0.5—4
Titanium oxide 0.5—3
Alkalies 1—4
Table 15 shows the concentration of seventeen trace metals before and after
collectors. 414
ThERMO. SYSTEMS INC.
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Table 15. Metals Analysi. for Full-Load Tests 414
(grainslsef x
Type of
Boiler
Firing
Avg.
Coil.
%
Ssinp.
Point
Cd
Ba
Be
Fe
Pb
Cr
Cu
Sn
Sb
Mo
Ni
Mo
V
Ti
Zn
Co
As
Vertical
89
B
A
T
T
9.5
0.3
0.24
0.02
‘8o
17
3.6
1.1
0.95
0.0 3
9.5
O.87
T
0.014
T
T
7.2
0.26
14.3
1.1
0.95
0.17
9.5
0.88
9
3.14
T
t .34
o. e
.06
Corner
94
B
A
<.142
<.0214
20.8
1.14
0.112
0.024
1900
102
11.2
0.21
8.3
0.58
25
1.1
<.142
<14.2
<.24
1’.a
0.1414
8.8
0.58
<1.22
0.10
214.8
1.11
1420
22
12.2
<.72
1.22
.0 .
1.14
0.11
Front-Wall
86
B
A
0.73
0.26
20.14
3.14
o.6o
0.1.1
1i80
58
125
1.2
14.8
o.68
3.6
0.88
<.26
0.26
<0.8
<0.8
1.7.0
1.6
12.5
.0.76
3.6
0.58
2.14
160
48
<24
c2.8
2.
0.37
Spremde -
Stoker
‘
B
A
T
T
3.6
0.20
1100
14.8
‘.4
1.95
1.52
1.9
1.1
T
0.17
<2.14
<0.14
6.1
1.3
3.6
1.5
0.73
0.37
6.1.
1.5
48
17
<7.3 ).73
3.0 0.21
2.6
1.6
Cyclone
69
B
A
0.30
0.12
27.2
7.5
0.28
0.08
1360
380
11.14
3.8
8.2
2.2
3.2
0.8
0.65
0.26
<1.14
<0.14
5.7
1.2
10.3
2.2
1.14
0.38
13.6
11.7
136
38
<14.2
<1.2
2.2.
0.13
o.6(
0.3’
Rorisontally
Opposed
814
B
A
<.97
o. 8
6.8
1.1
0.914
0.114
6800
730
68
14. -
9.7
1.8
20
4.4
<.68
0.32
<6.7
<0.8
10.6
0.73
20
3.0
10.6
2.2
112
6.6
5110
73
142
111
5.1
0.66
L.o
0.51
Baaed on particulate grain loading. Each value is the average of at least two tests.
B: Before fly-ash collector
A: After fly-ash collector
T: Trace; blank indicates no data.
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—34—
Carbon content of fly ash ranges from virtually zero to 55 percent.
Representative figures are about 2 to 20 percent, with an average of about
10 percent. 1 ° 1 - 9 Factors which influence the degree of carbon in ash include
character of the coal, fineness of pulverization, burner design, location and
adjustment, furnace temperature, coal—feed rate, and amount of excess air.
f. Solubility. Fly ash contains water—soluble sulfates whose content
varies from less than 0.1 percent to several percent. 1019 The average water
solubility is about 7 percent. 744 Some values of percent benzene—soluble
organics are listed in Table 13.
g. Toxicity. In past practice, fly ash has been considered to be non—
toxic, and overall acidic in nature.
h. Wetability. Fly ash is usually difficult to wet and is believed to
become more difficult to wet as the combustible content of the ash increases.
1. Bulk Characteristics. Fine fly ash of less than about 20 to 40 pm
has a pronounced tendency to form stable agglomerates or masses and also to
adhere to surfaces. Coarse fly ash acts more like sand and does not exhibit
these cohesive and adhesive tendencies. Some kinds of fly ash will cement and
form semi—hard accretions on surfaces if allowed to become moist or slightly wet.
Fume particles, which typically are less than 1 pm,show a high degree of cohesion.
The specific surface area for fly ash is usually between about 2,000 and 15,000
cm 2 1gm. 442,1019
j. Optical. Some glass particles in fly ash are light green or amber in
color while others contain inclusions of iron oxide, a birefringent material.
The index of refractiort for glass ranges from about 1.50 to about 1.64. The
lower index particles are generally clear or semiopaque due to bubbles; the
higher index particles are green or amber. The magnetite—hematite particles are
opaque in transmitted light and have a gray metallic luster in reflected
light. 442 ’ 1019
EMISSIONS FROM OIL FIRING
1. Variables Affecting Emissions From Oil—Fired Power Plants
Although particulate loadings from large oil—fired furnaces are usually
considerably lower than for coal, oil—fired loadings vary over a much broader
range. The major factors influencing particulate emissions in oil combustion
are efficiency of combustion and atomization. Combustion effici ency is lowered
by poor mixing, turbulence of the air and oil, low flame temperature and short
residence time in the combustion zone. 62 ° This results in larger particles,
higher combustible content and higher particle loadings.
ThERMO. SYSTEMS INC.
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The extent of atomization is strongly affected by the oil pressure. Low
pressure atomization of oil promotes larger fly ash particles and greater
particle concentrations; high pressure atomization yields smaller fly ash
particles, fewer cenospheres (hollow carbon skeletons) and lower particulate
emissions.
The extent of atomization is also affected by the temperature of the fuel
oil. Increasing the oil temperature decreases ts viscosity. In a 186—megawatt
plant, an increase in the oil temperature of 35 F above the usual range of 230 to
240 F halved the emission rate and reduced the combustible content of the emissions
by 15 to 17 percent. 1251
Other factors and circumstances influencing emissions are: vindbox air 620
admittance, burner tilt, excess air, flue gas recirculation, and soot blowing.
Varying the settings on the main and auxiliary air dampers, which effects
the windbox air admittance, caused pronounced effects on ash emissions in two
series of tests on a 186—megawatt plant.’ 25 In the first series of tests, the
main dampers were not completely opened, but the auxiliary controls were opened
quickly. This produced large increases in the fly ash loading and combustible
content, 1251 In the second series of tests, a much wider range of damper setting
was used. The fly ash loading did not rise as sharply as under conditions of the
first series of tests. The combustible content stayed essentially constant in
the second series of tests.
Increasing the amount of excess air usually decreases the fly ash loading
and the combustible content since more complete combustion results. In a series
of four tests,’ 25 ’ an increase in the oxygen content of the stack gases from
2 to 4 percent resulted in a seven—fold decrease in particulate loading.
Recirculation of flue gases into the firebox increases fly ash emissions
because of cooling of the flame. One investigator 1251 reported that an increase
in flue gas recirculation from 0 to 15 percent increased the fly ash emissions
100 percent in a 186—megawatt plant. The same investigation found that when some
flue gas was recirculated, the burner tilt affected emissions • With the burner
tilted zero degrees from the horizontal, fly ash emissions reached a maximum.
Hence, a burner tilted either up or down should be the best position from a
particulate emissions standpoint.
Sootblowing obviously increases the particulate emissions. For example,
this same investigatorl 25 l reported a 2.3X increase in normal emissions of
0.028 gr/scf during sootblowing.
Table 16620 shows the affects on total emissions due to an increase in
these variables.
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TABLE 16. Effects On Emissions Of Increasing Operating
a,620
Variables
Increasing Operating MO, SO 2 SO 3 Particulates
Variables — — —
Percent Load I — I
Fuel Temperature D — I D
Fuel Pressure D — I D
Excess Air I — I D
Percent CO 2 in Stack D — D I
DirtinFirebox I — I I
Flue Gas Recirculation D I
Flame Temperature I — I D
Stack Temperature — — I D
Percent Sulfur in Oil — I
Percent Ash in Oil — — D
a means increase; D means decrease; — means no change.
2. Particulate Loadings From Oil Combustion
Unlike coal combustion, the particulate emissions from oil firing do not
follow any trend when correlated with the ash content of the oil. When oil
containing one pound of ash is introduced into a larger boiler, as little as one—
half pound or as much as 10 pounds of particles could be emitted. 62 ° The emissions
may result from a build—up or detachment of boiler deposits, carbon and carbonaceous
matter in the fly ash, sulfuric acid reacting with the boiler stack, or from a
combination of these factors.
‘Most fuel oils contain less than 0.2 percent incombustible ash 526 which
appears in the stack gases as particles. Since 1 gallon of oil when fired equals
about 1760 scf of stack gas at 12 percent C0 2 , 0.2 percent ash content of the oil
corresponds to a stack gas loading of 0.065 gr/scf. Emissions cited in the
THERMO- SYSTEMS INC.
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literature, however, range from 0.008 to 0.49 gr/scf. 2 18 ’ 367 ’ 368 ’ 465 ’ 10 More
typical reported values are between 0.03 to 0.08 grlscf. Most of the values
reported in the literature are for small combustion units, but these values are
believed to be typical of large units as well.
3. Particle Size of Emissions From Oil Combustion
Particle size of sah in stack gases resulting from oil combustion usually
varies from less than 0.01 micron up to 40 microns in size. 526 The extremely
small particles are carbon produced by cracking of vapor distilled from fuel oil.
The larger particles, composed of carbon and ash and often in the form of hollow
spheres of lattice construction (cenospheres), are produced by crscking of fuel
in the liquid droplet state. 591 Under proper combustion conditions the carbon
can usually be consumed.
Sulfur trioxide gases condense on fine, free carbon particles causing them
to agglomerate to large flakes which may vary in size up to 1/8 or 114 inch
diameter. 45 1 Electron photomicrographs have shown that liquid fuels produced
sulfated ash particles predominantly in the 0.2 to 1.2 micron range. 107 l
It is believed that the time—temperature relationship of the products of
combustion determine the size distribution of submicron particles discharged
from the stack.
4. Control of Particulate Emissions From Oil—Fired Power Plants
The only pollution control devices used to any extent on oil—fired units
are cyclone collectors limited for use during sootblowing. Cyclones are highly
efficient for particles greater than 10 microns in size but are quite in-
efficient for particles 2 microns or smaller in size. The use of electrostatic
precipitators is presently very limited. However,when electrostatic collectors
are used, they not only reduce solid particle emissions by as much as 90 percent,
but also SO 3 emissions by as much as 50 percent. 62 °
5. Physical Properties and Composition of Particulate Emissions From Oil—Fired
Power Plants
A paucity of information on the physical properties and composition of
particulate emissions from oil—fired plants exists in the literature. Con-
sequently, this section contains far less information than the corresponding
section for coal—fired power plants.
• a. Particle Density. Density is normally about 2.5 gm/cm . Insufficient
information is available to determine a range for particle density.
b. Bulk Density. No information available.
TH ERMO- SYSTEMS INC.
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c. Particle Shape. Shape is extremely variable.’ 349 Generally, the
smaller particles tend to be spherical while the large ones form hollow spheres
of lattice construction.
d. Electrical Properties. No quantitative data available. However, the
resistivity of fly ash from oil firing is probably less than for fly ash from
coal combustion because of its higher carbon and sulfate contetw.
e. Chemical Composition. Fly ash is mainly carbon and ash. 124 ’ Depending
upon the source of the fuel oil burned, the ash will contain varying amounts of
silica, sulfate, and oxides of iron, aluminum, titanium, calcium, magnesium,
vanadium, nickel, and sodium. 62 °
f. Solubility. Solubility varies from 30—60 percent. Some values of
percent benzene—soluble organica are listed in Table 13.620
g. Toxicity. Fly ash is considered to be nontoxic but acidic, having an
initial pH of about 3.0.425,620
h. Wetability. Fly ash is difficult to wet and becomes more so as the
combustible content increases.
1. Bulk Characteristics. No information available.
j. Optical. No information available.
Characteristics and Properties of Stack Gases From Coal—Fired and Oil—Fired
Power Plants
Particulate emissions from power plants are measured in the ducting before
and after collectors or in the stack. The choice of station for measuring emissions
is governed by its accessibility and the expected flow conditions. The vertical
stack is usually the preferred location for sampling if it is accessible.
The flow conditions at any sampling station can be adversely affected by
bends, turning vanes, distributor plates, and expansions or contractions in the
ducting ahead of the sampling station. In most power plants, there are no
sampling stations where the flow conditions are nearly ideal (i.e., low mechanical
turbulence and a well defined flow profile). Figure 17 shows that the velocity
profile at the entrance to an electrostatic precipitator can be nonuniform, in
spite of careful design to ensure good flow conditions through the collector equip-
ment, a requirement for efficient collector operation. The velocity profile down-
stream of the collection equipment is usually even more nonuniform because most
ducts are short in length and contain elbows and turning vanes.
ThERMO SYSTEMS INC.
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13.0 17.3
Precipitator Entrance Sampling
Port Distance in Feet from Left
to Right Facing Port
0 Appr3ximat Sampling Port Location
Fig. 17. Velocity Profile in a Flue—Gas Duct Just Upstream of an
Electrostatic Precipitator. 334
From visual. observations, the intensity and scale of the turbulence appear
to be large. Also, the periodicity of flow fluctuation, judging from the response
of monitors installed in power plants and from observed fluctuation at sampling
ports, is estimated to be 0.5 to 2cycles per second. In addition, it is
estimated that the flow fluctuations are about 6 percent of the average flow velocity
ThERMO-SYSTEMS INC.
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110
100
90
80
w
6O
40
30
20
10
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—40—
The temperature of stack gases from larger power plants varies between
250°F and 400°F. In general, the flue gas temperature of oil—fired plants is
slightly higher than for coal—fired plants.
The acid dewpoint of flue gases from coal—fired plants is about 270°F and
the water dewpoint is about 140 F. The corrosivity of stack gas becomes a
problem when the temperature of an object in contact with the flue gas is below
the acid dewpoint. Corrosivity of flue gases from coal combustion reaches a
relative maximum at about 220°F. Below the water dewpoint of about 140 F, the
corrosivity rises rapidly with decreasing temperature. 428 No information has
been found about the, dewpoints of vapors in oil—fired plants.
Sulfur trioxide, free or combined, is perhaps the most corrosive agent
attacking metal surfaces exposed to flue gases. Sulfur trioxide formation
depends on several factors, including flame temperature, rate of effluent cooling
and the SO 2 — SO chemical balance, and is normally 10 percent of the sulfur
dioxide emiss on.398 Some values of the sulfur content of fuels and the
sulfur dioxide concentration in flue gases are shown in Tables 5, 7, 12, and
13.
Excess air (or oxygen) and carbon dioxide content of flue gases are
important since concentrations of particulate emissions are frequently reported
on a basis related to these factors. Some values of carbon dioxide content and
excess air are given in Tables 6, 8, and 13.
Typical values of the moisture content lie in the range of 5 — 10 percent.
Tables 6, 8, and 13 shows values ranging from 2 — 9.5 percent. The moisture
content affects the water dewpoint.
SUMMARY
Table 17 summarizes the effluent characteristics from large coal— and oil—
fired combustion facilities. The information presented is based upon information
obtained from a search of the literature, from visits to power plants, and from
discussionB with the operating personnel and engineers in power plants. Obviously,
much more information than is presented in Table 17 is needed to define completely
the environment in which a continuous mass monitoring instrument must operate.
However, to obtain more and better information requires better instrumentation
than is now commercially available.
A list of specifications for an acceptable instrument to monitor continuously
the mass emissions of particles from the stacks of Large coal— and oil—fired power
plants is presented following Table 17. The specifications are based upon the
information presented in this section, especially that presented in Table 17, plus
a knowledge of instruments and techniques that exhibit a potential for measuring
mass emissions of particles in stack gases.
THERMO- SYSTEMS INC.
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TAISL 17 SUMNAR OF EFFLUENT ChARACTERISTICS FOR COAL AND OIL COMBUSTION
COAL COMBUSTION EMISSIONS
(Controlled and uncontrolled as noted)
OIL COMBUSTION EMISSIONS
(Uncontrolled)
Par cu1ate Mass Concentration
(units of grams/cubic meter)* Before After
Collector Collector
Typical: 0.06 — 0.2
Range: 0.02 — 1.0
Pulverized fired 2.0 — 15.0 0.03 — 4.0
Stoker fired 0.2 — 10.0 0.06 — 3.0
Cyclone_fired 0.4_—_4.0 0.03_—_1.0
Mass Concentration Variations:
Spatially across duct
ith time
During sootbiowing
Typical: ± 5Q%
No information available
No information available
Typical: ± 50%
As much as 10—fold increase over typical
About 4—fold increase over
Particle Size
Extreme particle size range
Particle specific density
(units of microns
Before Collector After Collector
diameter)
Rg. of 5% of 95% of Mech. 95% Elect. 95
MMD** Mass less Mass Less of Mass of Mass
Than Than Less Than Less Than
.
Typical: 0.01 — 1.0 microns
Usually uncontrolled
0.01 — 40 microns diameter
0.5 — 5.0 range (variable within each stack)
Pulverized fired 7—20 1.0 70 40 25
Stoker fired 15—70 2.0 100 40 25
Cyclone fired 3—10 1.0 50 40 25
0.01 — 300 microns dian eter
0.5 — 10.0 gm/cm 3 (variable within each stack)
Flue gas velocity
Flow condition
Periodicity of flow fluctuations
Range: 30 — 120 fps Average: 50 — 60 fps
Average velocity fluctuates less than ± in most stacks
Turbulent and nonuniform
30 — 120 cycles per minute
Range: 30 — 120 fps Average: 50 — 60 fps
Average velocity fluctuates less than
+ 3 fps in most stacks
Turbulent and non-uniform
30 — 120 cylea minute
Flue gas temperature
Dew point of flue gases
Moisture content of flue gas
Range: 270 — 40 F Typical 290—330°F (varies less than ± 5°F
in most stacks)
Water: ‘140°F Acid: 220 — 270°F
5 — 10% by volume
Typical: Slightly higher than coal—fired
No quantitive information available
No information available
Existing sampling port size
Distance across duct from port
Static pressure at sampling ports
3 — 8 inch 1.11 hole typical
Range: 5 — 30 ft. Typical: 7 — 15 ft.
Range: 5” posItive to 15” negative water pressure
Typical: 0” to 10” negative water pressure
3 — 8 inch I.D. hole typical
Range: 5 — 30 ft. Typical: 7 — 15 ft.
No information available
.
*1 grain/cubic foot- — 2.29 grams/cubic meter
= mass median diameter of the particle size distribution
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—42—
INSTRUMENT SPECIFICATIONS
The specifications for an acceptable instrument (permanently installed) to
monitor the mass emissions of particles downstream from a control device on a
large stationary coal— and oil—combustion facility are as follows:
Performance Characteristics
• Measurement must correlate with total mass emissions of
particles into the atmosphere.
• Capability of sensing particle mass concentration in the
range of 0.01 — 4.0 grams per cubic meter.
• Must cope with particulate mass concentration profiles
which vary by typically ±50 percent spatially, by a
factor of about 10 with time, and by a factor of about
10 during sootbiowing.
Preferably senses the mass of particles from 0.01 to 300
microns. However, it would be acceptable if it senses
mass from 0.1 to 50 microns for coal—firing emissions or
from 0.05 to 10 microns for oil—firing emissions, which
would Include over 90 percent of the mass of particles.
Reproducibility between two identical instruments, including
both sampling and sensing error, of ± 20%.
Calibrated accuracy of ± 30%.
• Should record such items as zero and span every 1 to 6
hours on data recordings.
• Preferably have an instantaneous readout of the total mass of
particles emitted into the atmosphere per unit time, or at
least one average readout every 15 minutes.
• Outputs compatible with standard strip chart or digital
recorders and with computer data inputs.
Environmental Requirements
• Operate with flue gas velocities of 30 to 120 feet per second.
Operate in a variety of stacks, each having widely different
velocity profiles.
Not adversely affected by turbulent flow with characteristic
eddy dimensions of 6 inches to 6 feet.
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• Operate with flue gas temperatures ranging from 250 to 4000! and
with temperature fluctuations of ± 5 F for short range periods
(less than 1 day) and of ± 200! for long range periods (more
than 1 day).
• Operate in corrosive flue gas containing sulfur trioxide, both
combined and uncombined.
• Must not restrict the flue gas flow in any significant way.
• Must withstand such environmental conditions as vibrations with
amplitudes as high as 1/2 inch and frequencies on the order of
0.1 — 1.0 Hz, all types of meteorological conditions, and direct
sunlight. Must withstand environmental temperature variations
from +130°F to —30 F on a single installation.
• Must operate with flue gas static pressures from 15” negative to
5” positive water pressure (atmosphere reference) and with
fluctuations of ± 3” water pressure.
Maintenance and Operational Considerations
• Rugged enough to last several years without major repair.
• Little or no maintenance except for regular weekly maintenance
and for major maintenance only during regular power plant
maintenance shutdowns (usually every 6 months).
Little or no calibration or adjustment while in operation.
Easily accessible during and after installation for weekly
maintenance and for calibration or adjustments.
• Easy for plant operators to understand, operate, and to make
minor repairs.
Operate on llOV or 220V, 60 Hz electrical power and require
little power, preferably no more than 1500 watts.
• May use up to 10 scfm of 90 psig compressed air in many
facilities.
Economic Factors
• Installs easily within a week by 2 men in present and future
stacks with little stack modifications other than holes,
flanges, and utilities.
• Preferably cost $10,000 or less but definitely not more than
$20,000.
• Power industry personnel must be willing to buy it.
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REFERENCES
334 Anon., Power Plant Data (1970).
774 Anon., Stack Emission Reports.
239 Anon., Determining the Properties of Fine Particulate atter , American
Society of Mechanical Engineers, PTC—28—1965, 40 p. (1965).
788 Anon, “Control Techniques for Particulate Air Pollutants”, US. Public
Health Service, Nat. Air Poll. Cont. Admin., Washington, D ’. C., U.S.
Govt. Printing Office (1969).
422 Anon., “Information Required for Selection of Electrostatic and
Combination Fly Ash Collectors — Methods of Analysis far Chemical,
Physical, and Electrical Properties of Fly Ash”, APCA Journal , V. 15,
no. 6, p. 256—260.
526 Austin, H. C., “Atmospheric Pollution Problem of the Public Utility
Industry. Sources of Potential Air Pollutants”, APCA Journal , V. 10,
p. 292—294 (1960).
218 Brandon, J. H., “Can A Fuel Treatment Program Control Stack Emissions?”,
Combustion , p. 20—24 (Oct 1969).
590 Brown, R. 1., “Some Coal Research Problem and Their Industrial
Implications”, Journal Inst. of Fuel , V. 29, p. 218—236 (1956).,
465 Bunz, P., Neipenberg, H. P., and Rendle, L. K., “Influence of Fuel Oil
Characteriètics and Combustion Conditions on Flue Gas Properties in
Water—Tube Boilers”,Journa]. Inst. of Fuel , V. 40, no. 320,406—416
(Sep 1967).’
434 Busby, H. G T.,and Darby, K., “Efficiency of Electrostatic
Precipitators as Affected by the Properties and Combustion of Coal”,
Journal Inst. of Fuel , V. 36, no. 268, p. ’l 84 —l97 (May 1963).
413 Cuffe, S. T., Gerstle,R. W., Orning, A. A., and Schwartz, C.H.,
“Air Pollutant Emissions from Coal—Fired Power Plants — Report No. 1”,
APCA Journal , V. 14, no. 9, p. 353—362 (Sep 1964).
414 Cuffe, S. T., and Gerstle, R. W., “Emissions from Coal—Fired Power
Plants: A Comprehensive Siimm ry”, U. S. Public Health Service Pub. No,
999—AP—35, 26 p. (1967).
1055 Duprey, R. L., “Compilation of Air Pollutant Emission Factors”, Public
Health Service, Raleigh, N. C., Clearinghouse No. PB 190 245 (1968).
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1163 Duzy, A. F., “American Coal Characteristics and Their Effects on the
Design of Steam Generating Units”, Preprint, Am. Soc. of Mech. Engineers,
New York, 1959 (Presented at ASME Annual Meeting, Atlantic City, N.J.,
Paper 59—A242, Nov 29 — Dec 4 1959).
323 Elder, J. L., and Kube, W. R., “Technology and Use of Lignite”, Proc.,
U. S. Bureau of Mines—U. of N. D. Symp., Bismarck, N. D., Apr 29—30,
1965. U. S. Bureau of Mines Information Circular 8304 (1966).
365 George, R. E., and Chass, R. L., “Control of Contaminant Emissions from
Fossil Fuel—Fired Boilers”, Mn. Chem. Soc. Div. Fuel Chemistry , V. 10,
no. 1, p. 31—56, papers for meeting March 22—31, 1966.
407 George, R. E., and Chase, R. L., “Control of Contaminant Emissions from
Fossil Fuel—Fired Boilers”, APCA Journal , V. 17, no. 6, p. 392—395 (1967).
415 Ceratle, R. V., Cuff e S. T., Orning, A. A., and Schwartz, C. H., “Air
Pollutant Emissions from Coal—Fired Power Plants, Report No. 2”, APCA
Journal , V. 15, no. 2, p. 59—64 (Feb 1965).
1240 Gould, C., “Formation of Air Pollutants”, Power , p. 86 (Aug 1960).
857 Grohee, E. S., “Atmospheric Pollution: the Role Played by Combu8tion
Processes”, APCA Journal , V. 8, no. 3, p. 255—266 (Nov 1958).
368 Hangebrauck, R. P., Von Lelmden, D. 3., and Meeker, J. E., “Emissions of
Polynuclear Hydrocarbons and other Pollutants from Heat—Generation and
Incineration Processes”, APCA Journal , V. 14, no. 7, p. 267—278 (Jul 1964).
398 Hovey, H. H., Risman, A., and Cunnan, J. F., “Development of Air
Contaminant Emission Tables for Nonprocess Emissions”, APCA Journal ,
V. 16, 7, p. 362—366 (1966).
1251 Jeff erie, G. C., and Sensenbaugh, 3. D., “Effect of Operating Variables
from a Modern Power Station Boiler”, ASME , (Oct 1959).
608 Katz, J., “The Effective Collection of Fly Ash at Pulverized Coal—
Fired Plants”, APCA Journal , V. 15, no. 11, p. 525—528 (1965).
451 Kirov, N.Y., “Sulfur in Oil Fuels — Its Effects in Combustion”, Journal
Inst. of Fuel , V. 35, no. 261, p. 426—431 (Oct 1962).
749 McConnel, J. A., “Burner Fuel Oils Market Demand and Quality Require-
ments”, Sun Oil Company, Marcus Hook, Penn., presented at National Fuels
& Lubricants Meeting, New York New York, Sep 11—12, 1968.
1349 McCrone, W. C., Draftz, R. C., and Delly, 3. C., The Particle Atlas ,
Ann Arbor Science Publishers, Ann Arbor, Mich. (1967).
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442 Nature and Distribution of Particles of Various Sizes in Fly Ash:,
U. S. Waterways Experiment Station, Tech. Report 6 — 583, 27 p.
(Nov 1961).
1241 Negherbon, W. 0., “Sulfur Dioxide, Sulfur Trioxide, Sulfuric Acid
and Fly Ash: Their Nature and Their Role in Air Pollution”, EEl
Research Project RP62 , (Jun 1966).
367 Pesterfield, C. H., “Literature and Research Survey to Determine
Necessity and Feasibility of Air Pollution Research Project on
Combustion of Coninercially Available Fuel Oils”, APCA Journal , V. 14,
no. 6, p. 203—207 (Jun 1964).
425 Raco, R.J., and Peskin, R. L., “High Voltage, Low Frequency Atomization
of Fuel Oil”, ASHRAE Trans. , V. 75, Pt 1, Paper 2099, p. 111—117 (1969).
1071 Salo, E. A., “Visible Exhaust From Fuel Oils”, Hydrocarbon Process ,
V. 49, no. 2, p. 96—98 (Feb 1970).
591 Sambrook, K. H., “Fuel Oils, Their Efficient and Smokeless Combustion”,
Petroleum , V. 17, p. 56—61, 63 (1954).
401 Shafer, H. E., Jr., and Holland, C. T., “Western States Coal—Associated
Mineral Occurences Likely to be Factor in Long Range Air Pollution
Considerations”, West Virginia University School of Mines—Coal Research
Bur. Tech. Report 17, 13 p. (Oct 1965).
496 Shale, C. C., Holden, J. H., and Pasching, C. E., “Electrical
Resistability of Fly Ash at Temperatures to 1,500°F”, U. S. Bureau
of Mines, Publication 7041, 17 p. (1968).
946 Smith, V. S., and Gruber, C. W., “Atmospheric Emissions from Coal
Combustion — An Inventory Guide”, U. S. Public Health Service, Division
of Air Pollution, Cincinnati, Ohio, Clearinghouse No. PB 170 851
(Apr 1966).
620 Smith, W. A., “Atmospheric Emissions from Fuel Oil Combustion”, U. S.
Public Health Service, Pub. No. 999—AP—2 (Nov 1963).
428 Third, A. D., “The Aim of Chimney Design”, Eng. & Boiler House Rev. ,
V. 82, no. 5, p. 124—128 (May 1967).
727 Vandergrift, A. E., “Particulate Pollutant System Study”, Midwest
- Research Institute, Kansas City, Mo., Reports for NAPCA, U. S. Dept.
of HEW, Cincinnati, Ohio, Cont. No. CPA 22—69—104 (1970).
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189 Walker, A. B., “Emission Characteristics from Industrial Boilers”,
Air Engineering , V. 9, no. 8, p. 17—19 (Aug 1967).
1019 White, H. J., “Effect of Fly—Ash Characteristics on Collector
Performance”, APCA Journal , V. 5, p. 37—50 (May 1955).
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SAMPLING CONSIDERATIONS
by: John Borgos
INTRODUCTION
The extraction of a sample of particles from a gas stream, such as flue
gas in a smokestack, and the delivery of this sample to some sensing instru-
ment for evaluation is a formidable task. The degree to which the sample truly
represents the gas stream is always highly dependent on the design of the probe
used to remove and transport it. The designer of the probe must have the flow
conditions of a particular gas stream in mind to insure that theprobe gets an
unbiased sample of the stream. He must also have the specific type of particle
in mind to insure that, once the aerosol enters the probe, it is not modified
in such a manner that it affects the measurement.
A probe must have at least two main parts: a nozzle for sample removal and
a tube for sample transport. The nozzle must be sized so that it draws gas at
a velocity equal to that in the duct airstream and at a volumetric flow rate
compatible with that required by the sensing instrument. It must be shaped go
that it obstructs the bulk flow as little as possible. The tube must be sized
and fabricated to prevent excessive particle deposition and reenjrainment on
the tube walls. Bends have to be properly designed to avoid inertial deposition.
Heating, cooling, or, diluting of the sample may help maintain its integrity.
Thus, there are a number of criteria which the probe designer must satisfy to
optimize the design of the nozzle and sampling tube.
It must be borne in mind that perfect probes do not exist and, with present
information and materials, cannot be made. Therefore, the optimization of the
probe design is only half the solution to the problem of getting a sample from
the gas stream to the sensing instrument. The other half is an evaluatioji of
the errors which cannot be removed in the probe design. Such evaluations are,
at best, intelligent guesses, but they do give the order of magnitude of the
errors.
The remainder of this discussion will be limited to the sampling of flue
gas from ducts or stac±s. A general review of the conditions expected in a typical
stack is included. The significant sources of error are discussed in terms of the
parameters that govern them. From this, criteria for good probe design develop,
Finally, the magnitude and importance of the expected errors are evaluated with
specific particle sensing instruments in mind.
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STACK GAS PARANETERS
The authors have conducted an extensive study of the literature on the
properties of flue gas from coal— and oil—fired combustion sources. The
assimilation and condensation of all this information, which is presented in
a previous section of this report, has provided a comprehensive picture of
what is now known about the gas flow and particle properties of flue gas. One
of the first lessons learned from this Btudy is that each of the variables has
a wide range of values. Therefore, any instrumentation has to be designed for
use over a wide range of conditions.
Before going into a detailed discussion on probe design features, a more
general presentation of the relevant gas flow and particle properties will be
given. Although data is available for conditions both upstream and downstream
of particle collectors (e.g., cyclones and electrostatic precipitators), attention
will be focused primarily on conditions downstream of the collectors. The
reason for this is that a measurement of particles here defines what is actually
going out of the stack, and thus what may become air pollution.
There are a couple of disadvantages in monitoring conditions downstream, as
opposed to upstream, of the particle collectors. First, the gas flow is usually
more turbulent downstream of collectors than upstream. This is due perhaps
mainly to the fact that particle collectors require a fairly stable flow con-
dition, and therefore an effort is made, either with a straight section of
breeching or a series of guide vanes, to provide uniform, stable flow in the
collector. This is not done downstream, however, because the chimney follows
quite closely behind the collector; thus uniform flow conditions do not have
time to develop between the collector and the chimney. Second, the particles
downstream of an electrostatic precipitator are often electrically charged.
This affects their behavior somewhat in a sampling probe and significantly in
most sensing instruments. The magnitude and distribution of the charge are
unpredictable. Therefore, a well designed instrument should not be affected
by this residual charge possessed by the sampled particles.
The level of turbulence in flue gas ducts and the resulting sampling
problems have not really been assessed. If the particles are not all flowing
in the same direction, or if they are not moving in the same direction as the
local gas, the orientation of a probe nozzle is critical. Indeed, if the
velocity vectors of the particles in the duct are not unidirectional, a severe
sampling bias may occur regardless of the nozzle orientation. The mean velocity
of flue gas in ducts is usually about 15 — 20 m/sec, which means the Reynolds
number varies from 106 — l0 for typical ducts (based on the duct diameter).
The normal frequency range of flow fluctuations appears to be around 0.5 — 2 Hz.
Thus, large scale (about a meter in diameter) turbulent eddies are likely to
be present. These eddies help distort the distribution of the particulate mass
loading across the duct, which probably varies by as much as ± 50% from one
position to another.
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A second factor contributing to the skewed distribution of particulate
mass loading across a duct is the duct configuration, which contributes also to
the formation of turbulent eddies. Right—angle bends, for instance, cause large
particles (i.e.,>5 im) to be hurled toward the outside of the bend because the
lérodynamic drag forces are not great enough to overcome inertial forces. It is
usually recommended that about ten diameters should be allowed downstream of
a bend for the gas f low to become uniform. Unforti.uiately, few affluent ducts
have any straight sections which are this long. Worse yet, since the particle
eddy diffusivities are always smaller than those of the gas, the particles will
not become uniformly distributed as quickly as the gas flow. Therefore, bends in
the duct configuration are likely to contribute a great deal to the distortion of
the particulate mass loading distribution as well as the distortion of the
velocity profile across a dust cross—section.
A third contribution to nonuniform particulate mass concentration across
a duct can be made by the malfunction of various parts of the electrostatic
precipitator. Suppose several electrodes in a localized region of the pre-
cipitator are not functioning. The result is a heavier concentration of particles
in that part of the flue gas stream. The difference between this concentration
and the average concentration may be as much as one order of magnitude immediately
downstream from the precipitator.
INTEGRATED VS. POINT MEASUREMENT
Before comparing the virtues of so—called “integrated” and “point” measure-
ments, the definitions of each must be clear. Measurements made by all techniques
which require extraction of a sample from the duct are called “point measurements”.
Point measurements are not made at a point in the true sense of the word, but are
averages over a small area defined by the sampling nozzle. The measurements are
nearly always the average concentration of particles at the nozzle entrance. Use
of this measurement as an estimate of the average concentration across the entire
duct requires knowledge of the particle concentration profile. On the other
hand, to estimate the flux of particles passing through the duct, one must also
know the velocity profile within the duct. Mathematically, the total particulate
flux F passing through the duct can be expressed as:
F = fc(A)v(A A. (1.)
Note that particle concentration C(A) and velocity V(A) are both functions of the
specific location within the cross—sectional area A of the duct.
In practice, concentration and velocity profiles can be handled in two ways:
1) by measuring the concentration and velocity continuously at a number of points
within the duct cross—section; or 2) by measuring the concentration and velocity
profiles once, locating the sample extraction nozzle at a representative location
within the duct cross—section, and assuming that the concentration and velocity
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profiles remain constant. With the first method, Eq. 1 is approximated by:
F= £ 1 C V 1 A (2)
where the subscript i denotes each of the measured points and is the portion of
the duct cross—section represented by measurement i. With the second method, Eq. 1
is approximated by:
F KCVA (3)
where:
C is the measured particulate concentration,
V is the average velocity across the duct cross—sectional area A, and
K is a proportionality constant relating the average duct concentration
to the concentration at the sampling point.
The value of K is determined for any sampling point from initial concentration
and velocity profile measurements.
The most commonly—used particulate monitoring technique for stacks at the
present time (light transmission) does not require sample extraction, but simply
measures the attenuation of a light beam shining through the duct. Measurements
made by transmissometers are often called “integrated” measurements. The term
“integrated” measurement, however, can be confusing since these measurements give
simply the average concentration (actually optical density) of particles within
the light beam. Use of this measurement as an estimate of the average concentration
within the entire duct cross—section requires knowledge of the particle concentration
profile, just as with extracted sampling. To estimate the flux of particles passing
through the duct, one must also know the velocity profile within the duct, again
just as with extracted sampling. The second method outlined above for locating
the sampling nozzle of a “point” sampler must also be used for locating the path
of a transmissometer light beam. If this procedure is used, Eq. 3 approximates
the total particulate flux passing through the entire duct. Locating of the
measuring beam of a transmissometer is just as important as locating the sampling
point of a point extraction sampler.
The ideal situation would be to measure the total mass flux of particles
passing through the entire cross—section of the stack. However, there is no way
to do this in practice. The only practical methods of measuring the particulate
flux are to 1) measure the average concentration across a diameter of the stack
(so—called “integrated” measurement) or 2) measure the concentration at a point
within the stack (so—called “point” measurement). In addition to the concentration
measurement, both methods require additional knowledge of the velocity and con-
centration profiles within the cross—section of the stack at the measuring location.
Thus, neither method has an obvious advantage from this standpoint.
The advantage that an optical tranamiasometer measurement does have over a point
measurement is that the transmissometer does not require sample extraction. As will
be explained in the section on “Light Transmission”, however, transmissometers do
not sense the mass of particles, a severe handicap if mass is the parameter of
interest.
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The transmissometer can measure the average particle concentration across
a stack diameter. However, the object in stack monitoring is not to measure the
aterage particle concentration across a stack diameter, but to measure the flux
of particles across a stack cross—section . Both “integrated” and “point” con-
centration measurements require similar information about velocity and concentration
profiles to relate the actual measurements to the desired particle flux. Thus, it
Is not clear at this time whether one or the other Qffers significant advantages
f or measurement of particle flux. The question may be academic for this study
because all mass sensing instruments require extracted, point samples.
EXTEACTIVE SAMPLING
The aim of this discussion is to develop guidelines for good probe design and
to apply these guidelines as specifically as possible to the most typical stack
sampling situations. The guidelines are based on the parameters of the flue gas
and particulate matter that were discussed above, since it is these parameters
which affect the removal and transport of the particles in the flue gas. The
removal of the sample from the gas stream will be considered first (i.e., the
nozzle). Following this the transport of the particles out of the stack (i.e.,
the tube) will be discussed.
It is commonly stated that in order for an extracted aerosol sample to represent
what is actually in the flue gas stream, the gas must be withdrawn isokinetically.
In other words, the velocity of the gas inside the Sampling nozzle must be equal to
the velocity of the gas outside the nozzle. Unfortunately, this is a practical
impossibility in stack sampling because of the random fluctuations of the bulk
velocity of the flue gas. In addition, turbulent eddies cause the local velocity
to change, not only in magnitude, but in direction as well. Even if it were
possible for the nozzle to follow the eddy fluctuations, the sample would not be
representative because particles in the gas do not precisely follow the eddies.
Thus, the term “isokinetic” becomes rather meaningless.
The problem of getting a good sample is now twofold: first; to get the “best”
possible sample; and second, to find out how representative that sample is.
NOZZLE DESIGN
The nozzle diameter that is chosen for a particular situation depends on
several variables. If the probe is sampling for an instrument that requires a
certain volumetric flow rate, the nozzle size is defined by the ‘required flow rate
and the flue gas velocity. Such is the case with most particle sensors. For
ordinary filter sampling, the flow rate through the filter may not be nearly as
critical. In such cases the nozzle diameter can be more arbitrary, and may be
limited only by the capacity of the suction pump.
The probe nozzle diameter should always be made as large as the other con-
straints permit, within reasonable limitation. Badzioch 829 has shown that the
effect of increasing the nozzle diameter is a decrease in the diáturbance of the
free’gas stream. The result of this is a more representative sample. The magnitude
of the improvement in changing from one nozzle to another cannot be determined with-
out additional data on the turbulence characteristics in flue gas streams.
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White].ey and Reed 868 have studied various nozzle shapes 1 A sketch of
each nozzle they tested is shown in Figure 1. The results of some data taken
in a wind tunnel under controlled conditions showed that, for both fine and
coarse dust, nozzle 1 gave the most accurate results. Neither the length of
the nozzle nor the chamfer angle seemed to cause any error, so for their
particulate situation nozzles 2, 3, 4, and 5 were nearly as accurate. Probe 7
gave s1i htly higher errors, while probe 6 was very much in error. Lundgren and
CalvertSU 8 have also experimented with side port probes. Their results show that
only limited accuracy (approximately 20% error for 5 um particles) can be achieved
with such probes.
It is felt that the nozzle should be kept fairly long (at least two diameters)
so that particles enter the nozzle parallel to its axis. This will reduce losses
by inertial deposition within the nozzle and by the nonrepresentative entrance of
the particles into the nozzle. In addition, small chamfer angles (approximately
12° — 150) are desirable because they limit the accumulation of dust.
120° Chum fer
/80
(No chc mFer) (lMi 1 ongi )
- ® __
I j?.pp g_probes
_____A 0 /200 Chamfer
(rnc/uo’ed angi.)
1’ Short probes
hole iigbJ_ probes
____ oti probes mode From ‘ 0.0. x j i. A bross tubing
Fig. 1. Various configurations of sampling nozzles tested
with coal—combustion particles. 86 ö
0
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Although the turbulence in flue gas streams does not permit truly isokinetic
sampling, it is important that the velocity of the gas in the probe nozzle be
kept as close as possible to the mean velocity of the flue gas. The flue gas
velocity need not be monitored continuously, but it should be checked frequently
to avoid errors due to low—frequency fluctuations. Several methods can be used
for this.
Perhaps the most convenient means of keeping the velocitz adjusted properly
is the use of “zero—pressure” or “null—balance” probes. 257 ’ 95 These are probe
nozzles that are constructed such that the static pressures of the main and
sampled streams can be monitored (Figure 2). They are usually operated such that
the pressures are equalized and it is assumed that under these conditions the
two velocities must be equal also. Unfortunately, these conditions do not in fact
assure that the velocities are equal. Dennis et al 955 have shown that errors in
the order of 5% should be expected due to the interference caused by the tip of
the nozzle, and that this accuracy is possible only after a calibration. If such
a probe is used in flue gas, the accumulation of particles on the nozzle over a
period of time will no doubt change the calibration. Furthermore, the turbulence
in flue gas ducts, which is characterized by changes in the direction of the gas
velocity, would certainly make the error significantly greater because the static
pressure measurements are directionally sensitive. Such nozzles are therefore
not acceptable for use in stack gas.
____ P
Fl P 1 —
Direction
Fig. 2. Schematic diagram of a “null—balance” sampling nozzle.
\
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Groutsch’° 60 has built a zero—pressure device that has two nozzles. One
of the nozzles is the sampling nozzle. The other is a short section of pipe
which is open at both ends. It has a venturi throat which is designed such that
the differential pressure between the venturi throat and ambient is equal to the
differential pressure between the interior of the sampling nozzle and ambient when
the sampling flow rate is isokinetic. Unfortunately, this technique is not
feasible for stack monitoring becuase it would have the same directional sensitivity
and particle accumulation problems as conventional “null—balance” probes.
Boothroyd 256 has sati8factorily used a hot—wire anemometer to monitor the
flue gas velocity. This technique involved placing the hot—wire sensor in the
flue gas stream and noting the voltage. Then the probe nozzle was quickly placed
just downstream front the sensor and the aspiration rate was adjusted so that the
voltage on the hot—wire sensor equalled the previously measured voltage. The
velocity—voltage relationship of the hot—wire anemomter does not remain constant
for a long period of time because of contamination by particles in the gas.
Therefore, measurements cannot be made for longer than a few minutes. Furthermore,
the fact that the probe must be removed and then replaced. for every measurement
makes this technique undesirable for continuous stack. gas monitoring.
Another interesting technique has been tested by Grindell 122 . A sketch of
the apparatus is shown in Figure 3. It operates on the principle that the exit
Fig. 3. Schematic diagram of an aspirating probe u ing a
suction cone to replace the vacuum pump. 12 ’
cone will produce enough suction to overcome frictional pressure losses in the
system and small pressure drops in the sensing instrument. The suction is created
by the flow of the flue gas around the cone, which is designed such that the
nozzle samples essentially isokinetically. The results reported from tests in
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a wind tunnel showed that the instrument did indeed sample nearly isokinetically.
However, it did not operate as well in a stack, probably because of the increased
level of turbulence. These tests were not conclusive, and therefore this technique
should be investigated further. If it works, it would eliminate the need for an
aspirating pump. This is a major advantage because most pumps do not last Long
in flue gas. In addition, such a probe would respond automatically to changes in
the flue gas velocity, eliminating the need for elaborate pressure sensors or
flow controllers.
The use of a pitot tube which is separate from the sampling probe is the
most reliable technique for mo i oring the flue gas velocity. These have been
covered in detail by Falgout. 1 ’ 3 In particular, type pitot tubes offer
the best combination of sensitivity and reliability (Figure 4). The chief
advantage gained by the use o such devices is that they are nearly omnidirectional
for a11 (i.e., less than 10 ) angular velocity deviations. They are also less
sensitive to contamination by particles because they have much larger sensing
areas than, for instance, “null—balance” probes. If it is important to measure
small changes in velocity, “5” type pitot tubes offer a gain in sensitivity
because they measure the difference between the stagnation pressure and the
suction pressure. Pitot—static probes, on the other hand, measure the difference
between the stagnation pressure and the static pressure. Since the difference
between stagnation pressure and suction pressure always changes more for a given
velocity change, “S” type pitot tubes are more sensitive. Unfortunately they
require calibration before use, but because of their sensitivity and reliability
they should be used when monitoring stack gas velocities over extended periods of
time.
Fig. 4. Schematic diagram of an “5” type pitot tube.
FLOW
DIRECTION
h
4,
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The particle losses caused b anisokinetic sampling flow rates have been
studied by several investigators.” 9 ’ 829 ’ 1130 4229 Since the selection of the
nozzle inlet diameter is beyond the scope of this discussion (it is based on
the choice of the particle sensing instrument), a diameter of 3 cm will be
assumed. This size is typical of the majority of sampling probes of interest.
Badzioch 719 has calculated the errors from anisokinetic sampling of fly ash with
characteristics which are fairly representative. Given the above nozzle size, one
finds that the error in the particulate mass concentration measurement is roughly
60% of the error in the velocity measurement. Since average velocity measurements
in stacks are only accurate to within ± 5% at best, an error of at least ± 3% has
to be expected in the concentration measurement. In addition, although little is
known about flow conditions in stacks, a great deal of turbulence is expected.
This has not been taken into account in the error estimates. Therefore, a more
realistic estimate of the error caused by anisokinetic sampling and losses in the
nozzle would be 5 — 10%.
S LING TUBE DESIGN
The design of the sampling tube and the estimation of the errors due to
particle deposition are considerably more complicated. The tube characteristics,
flow characteristics, and particle characteristics are all important. Particles
can be lost by inertial impaction (this normally occurs around bends in the tube),
electrical charge effects, thermophoresis, gravitational settling, Brownian
diffusion, and turbulent eddy diffusion. The loss of particles by electrical
charge effects and thermophoresis is assumed to be negligible for purposes of this
discussion, assuming metal tubes are used that are at the same temperature as the
particles. The other effects are all significant in most situations.
Gravitational settling provides the smallest contribution to particle losses
because of the particle sizes an 1 flow ranges involved. Most of the mass of
particles in flue gas is contained in particles smaller than 25 pm. For typical
flue gas particles in a horizontal tube with a length of 3 m, a diameter of 2 cm,
and an average velocity of 9 m/sec, only about 6% of the mass of the particles will
be last by gravitational settling. The error due to gravitational settling is
proportional to
E LD 2 /UD,
S p
where
L = tube length,
— particle diameter,
U average gas velocity in the tube,
D = tube diameter, and
E 5 = error due to gravitational settling.
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For a given particle size, volumetric flow rate, and tube length, the error
varies directly as the tube diameter; i.e.,
LD D/Q,
where
Q volumetric flow rate.
Inertial impaction causes severe particle losses if the flow has to change
direction or if the fluid rotates about the axis of the tube. The deposition of
particles in curved sampling lines has been studied by Sebmel and Schwendiman
(Ref. 801, p. 88). These tests show that for both laminar and turbulent flow,
deposition increases with fluid velocity, decreases with probe diameter, and is
highly dependent on particle accommodation to the probe surface. Re—entrainment
of the particles depends on the stickiness of the particles and the smoothness
and stickiness of the tube wall. These effects are very difficult to assess
because little data is available on them and that which is available does not
apply to the particles in flue gas. It is a reasonable assumption that particle
re—entrainment will be significant for the larger particles in flue gas
(typically >15iim), which may make up as much as half the total particulate mass.
Therefore, any error estimate that ignores re—entrainment may isteif be in error
by as much as 50%. By making the probe diameter as large as possible, thereby
decreasing the fluid velocity, it is felt that the sampling error due to inertial
impaction around bends can be reduced to about 10%.
The impaction of particles by fluid rotation is usually not serious. For
example, in a 3 cm diameter tube with the ‘gas rotating at one revolution per
second and moving with a mean velocity of 6 m/sec, the velocity of deposition
of particles due to centrifugal action is nearly an order of magnitude less
than the gravitational settling velocity. Guide vanes would eliminate this
error altogether, and should be used if they can be conveniently installed.
The deposition of particles in tubes by Brownian and turbulent eddy diffusion
has received a good deal of attentior4 33 ,l 34 , 774 , 8 Ol,1173,1l75 The error caused
by eddy diffusion can be affected greatly by the re—entrainment of particles once
they deposit on the wall. Thus, there is a strong dependence on tube wall and
particle surface properties. Both brownian diffusion and eddy diffusion are very
dependent on particle size. Brownian diffusion is important only for particles
smaller than about 1 — 3 tm, whereas eddy diffusion is important only for particles
larger than this size.
The deposition of particles by eddy and Brownian diffusion is usually described
by a deposition velocity. This can be thought of as the average velocity of
particles toward the wall of the tube. It is equal to the mass (or number) of
particles deposited per unit area per second per unit particle concentration in the
air over the surface, and has the dimensions length per unit time.
111 ERMO- SYSTEMS INC.
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Davies’ 33 ” 34 has derived expressions that can be used to calculate the
deposition of particles due to the combined effects of Brownian and eddy
diffusion. These equations predict a strong dependence on particle size with
somewhat less dependence on the friction velocity, defined as the square root
of the ratio of the local shear stress to the fluid density. The friction
velocity affects only eddy diffusIon and is a measure of the turbulence Intensity.
As the friction velocity increases, the deposition due to eddy diffusion increases.
Wells and Chamberlain 11 ’ 5 have compared this theory with some experimental data
(Figure 5). While this does not show clearly the dependence on the friction
p .’
C)
w
bO
Fig. 5. Deposition velocity as a function of particle size showing
the particle—size dependence of Brownian and turbulent eddy
diffusion. 1175
velocity (denoted by ut), the dependence on particle size is made quite clear.
A minimum deposition velocity is seen to occur at a particle size of about 1 — 4 vim.
A similar behavior should be expected In sampling lines containing flue gas.
The particles sizes and tube diameter used by Wells and Chamberlain 1175 are fairly
representative of sampling lines. These tests also revealed a strong dependence
on the surface roughness of the tube wall. The roughness may be on a very small
scale such that it does not affect the turbulence level of the airflow. In fact,
it was found that the deposition velocity could be increased by more than an order
of magnitude by simply lining the tube with a fibrous filter material. It appears
that if particles begin to accumulate on the surface of a “smooth” tube, the
deposition characteristics of the tube may change over a period of time. This is
quite serious if. one is trying to correct for losses in the sampling tube during
a long test or a continuous monitoring operation, since the correction would be a
function of time. No theory has been suggested that takes Into account the minute
roughness. characteristics of the walls of the tube.
Particle diameter (pm)
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Brownian diffusion will cause significant particle deposition only for
particles less than about 0.1 urn. Thus, the importance of Brownian diffusion depends
on the particle sizes found in flue gas. The majority of emissions data leads to
the conclusion that most of the particulate mass from coal—fired sources is con-
tained in particles larger than 0.1 pm, and hence to the conclusion that Brownian
diffusion makes only a small contribution to the loss of particle mass.
On the other hand, turbulent eddy diffusion becomes significant for particles
larger than about 3 pm under typical sampling conditions. Since it causes the
particle deposition velocity to increase rapidly with particle size (it appears to
vary with something between — D ), its effect is important.
The prediction of particle losses is greatly complicated by the re—entrain-
ment of particles once they have come 4i contact with the wall of the tube. This
phenomena has been observed by Sehmel 77 ”, who measured radial concentration profiles
in a tube for various degrees of wall stickiness and particle sizes. As shown in
Figure 6, for 12 pm particles the concentration was a mini uin near the tube wall
0
Symbol Flagged
Symbol Unflagged
0.5 0.6 0.7
78
0.8 0.9
—
/
I
I
A
‘I
1.0 1.1 1.2 1.3
a
6
Normalized Filter Loading per Unit Area
Fig. 6. Particle concentration q a function of tube radius for smooth
and tacky tube surface.’’ 4
1.0
0
/
x
0 (0
0
Tube With
Tacky Surface
‘C
0.8
0.6
0.4
0.2
0
‘C
Filter Pressure
D ro p
0•
a
Inch
Water
3
Particle Size.
microns
x
12
A
28
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when the surface was coated with a sticky substance, whereas it increased close
to the wall when the surface was smooth. Also, for 28 pm particles, the con-
centration was 30% greater near the wall than at the tube center for a smooth
tube. Such concentration profiles can only be explained by the proposition that
the particles tend to stick well to the coated tube wall while many are re—entrained
into the gas stream from a smooth vail.
The re—entrainment of particles from tube surfaces does not lend itself to
theoretical treatment, Sehinel and SchwendhnnnSøl(p. 106) have performed experiments
with a variety of particle sizes, tube sizes, and. air velocities. From these
studies of radial concentration profiles and deposition velocities along the length
of the deposition surf ace, it appears that some of the particles that contact the
wall do not remain attached to it. They may either bounce off because of their
momentum or be swept away by the turbulent eddies that penetrate the laminar sub—
layer. Since the momentum of particles is proportional to D 3 and the adhesive force
is probably proportional to D the probability that a parti 1e will bounce is
nearly directly related to thR square of its diameter. It also depends on the
turbulence intensity, for this determines the particle velocity and hence its
momentum, However, since the fluid drag force is proportional to D 2 , the probability
of a particle being re—entrained after it is attached to the wall i 8 probably
less dependent on particle size than the probability that it will become attached to
the wall in the first place. Once the particle is re—entrained, it seems to remain
quite close to the surface, as evidenced by the concentration profiles observed.
This may be due to the acquisition of a charge by the particles from the tube wall.
If all the particles pick up a like charge, the resulting space charge should force
the particles outward toward the wall.
Particle re—entrainment is a phenomenon that is difficult to predict even
under ideal conditions, let alone in a sampling probe aspirating flue gas. The
available data attests to this, as it shows considerable scatter. Figure 7 shows
the relationship of the particle diameter to the deposition velocity for mean gas
velocities of approximately 2 and 12 rn/sec. The significantly lower deposition
velocity for the lower Reynolds number verifies the dependence on the turbulence
intensity. The initial positive slope of the curves is caused by the rapid increase
in deposition with particle size in a regime where re—entrainment is not allowed to
take place. Eventually, a size is reached where the particles possess enough
momentum to bounce off the walls of the tube, and further increase the particle
size causes a reduction in the net deposition velocity. Figure 8111 shows more
clearly the dependence of re—entrainment on particle size. Notice that the
deposition velocity may vary over several orders of magnitude.
For a given volumetric flow rate of air, the turbulent deposition velocity
always decreases with increasing tube diameter. This is because the Reynolds
number varies inversely as the tube diameter, and turbulent deposition is a strong
function of the Reynolds number. Therefore, the tube diameter should be increased
as much as possible to minimize turbulent deposition. However, it was shown that
the sedimentation deposition rate was directly related to the tube diameter. This
places an upper limit on the tube size, which must be chosen such that the total
deposition is a minimum.
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—
- 5
I0
20
25
liranlnc Particle DIameter. microns
Fig. 7. Deposition velocity versus particle diameter showing
the effect of re—entrainment. 801
As an example, assume a sampling nozzle with 2 cm diameter is used to aspirate
the flue gas, and that the mean gas velocity in the duct is 20 rn/sec. This fixes
the volumetric flow rate through the tube. If the tube diameter is 2 cm (which is
typical of aspirating probes), the mean velocity in the tube is also 20 m/sec,
corresponding to a Reynolds number of about 13,000. Taking the particle size to
be around 10 im (which is the particle size above which re—entrainment will occur
at this Reynolds number, as seen in Figure 8), one can estimate the deposition
velocity to be 0.2 cm/sec. Then, for a tube length of 3 m, one can calculate the
diffusion loss by the following equation:
in — 4 (jj )
I
.1 I I I
0
101 I I I I I I LLj F JI •
0 3
17.5 ft 1mm
Re . 2 ,800
•1 —
0
6 ’
0
0
i01
10-2
A
-0
U
0)
VI
E
U
‘. 1
U
0
V
C
0
V I
0
V
A
0
6
0
/
/
3.0
A Re
ft 3 ImIn
4190
A
• St
A A A
A
A
15
ThERMO-SYSTEMSIf4C.
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100
—6
0 1.0 2.0 3.0. 4.0 1.0 6.0
Reynolds Number x i0
Fig. 8. Deposition velocity as a function of Reynolds number and
particle size. 1111
C
C
0
V
U=
m
L=
D=
the particle concentration at the tube exit, gm/rn 3 ,
= the particle concentration at the tube entrance, gm/rn 3 ,
turbulent deposition velocity, cm/eec,
mean gas velocity in the tube, cm/eec,
tube length, cm, and
tube diameter, cm.
10
1
U
101
U
0
10-2
>
0
‘I
0
IO3
where:
7.0
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With the numbers given above, the concentration ratio, C/C is 0.94, so the
error is 6%. The error due to gravitational settling was calculated previously
for these conditions and found to be about 6%. Thus, under these sampling
conditions, 2 cm is probably the optimum probe diameter. A significant reduction
in size would increase the inertial losses around bends, and an increase in size
would make sedimentation losses too great.
It is quite apparent by now that the losses In sampling probes, including
the tube and the nozzle, will have to be taken into account In the interpretation
of any data. By simply adding the contribution made by the various loss
mechanism, one finds that the expected error lies somewhere around 30%. This error
will almost always be negative because particles are always lost, not gained, with
the exception that a low aspiration flow rate may increase the concentration of
large particles somewhat. Thus, If the various parameters were somehow known
the measurement error could be greatly reduced, probably as low as ± 15%.
As stated earlier, particle losses caused by thermophoresis effects have not
been determined because no reliable evidence has shown it to be significant.
Electrical charge effects may be a major factor since the sampling probe is
usually downstream of an electrostatic precipitator. However, almost no information
exists regarding the electrostatic charge level on the particles. Several investi-
gators have reported losses apparently caused by electrical effects.
Since particle deposition on sampling tube walls has been a serious problem
in so many aerosol measurements, a probe has been invented that is designed to
eliminate It. This is known as the boundary layer diluter probe. 1156 Although a
number of variations exist, they all operate on essentially the same principle
(see Figure 9). A boundary layer of clean air is introduced, either at the
entrance to the tube or all the way along the tube, which serves to keep the
particle—laden air away from the wall. The particles must therefore get through
this layer of clean air to be deposited. Their outward radial motion is opposed
by the Inward motion of the clean air if the diluter has a porous wall. The
boundary layer should be kept laminar as much as possible because any turbulent
mixing nullifies part of the advantage of the clean air.
There are two problems with boundary layer dilution. The first is that
normally the Reynolds number in the sampling tube is too high for a laminar
boundary layer to exist. Thus, some turbulent mixing of the boundary layer
will occur in the tube, though the deposition of particles will be somewhat
decreased.
The second problem arises because the flue gas must eventually be mixed
with the clean air, since many sensors require a homogeneous mixture of the aerosol
to give an accurate reading of the concentration. A significant percentage of
particles may. be lost, not only in the turbulent mixing process itself, but also
downstream of the mixing, where there is no boundary layer of clean air. Some
good data evaluating the effectiveness of these probes would be useful, however.
ThERMO . SYSTEMS INC.
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t t LLT D OSOL
TO COU. 3T
TU DULENC I DUC R
NOT REPRODUCIBLE
CLEt N
DILUTION G1 S
CYU ’ R
AEROSOL ST AM TO CZ DLUTW
T FROM STACK
Fig. 9. Schematic of a tube with boundary layer dilution.
G S ITPL J\ L
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Boundary layer dilution does offer the advantage that the flue gas is cooled
without condensation of the water vapor and acid. This may be important in some
cases where a sensing instrument cannot be made to operate at elevated temperatures
or where the conditions of the flue gas in cooLed, diluted form is of interest.
The flow rate capability of the particle—sensing instrument always has to
be considered when the sampling probe is designed. Instruments are available
that can handle from as little as one liter per minute to as much as several
hundred liters per minute. If an instrument can aspirate 300 liters/mm or more,
a sampling nozzle can be made that will get a fairly representative sample under
most conditions. If the flow rate is less than this, or if the sample is diluted,
an arrangement such as that shown in Figure 10 should be used. This makes it
possible to remove a relatively large amount of flue gas from the stack. Then,
once the velocity is reduced by increasing the tube size and the turbulence is
somewhat damped out, a smaller sampling probe can be used to aspirate the proper
quantity of gas for the particular instrument being used. It is felt that such
a configuration will provide a much better sample than a small sampling probe
mounted directly in the stack.
This design would not allow the use of a boundary layer diluter in the large
tube unless the stream was mixed thoroughly before it got to the small sampling
probe. However, the creation of turbulence would defeat the purpose of the system,
which is to damp out the turbulence so that a small probe can be effective. Thus,
bbundary layer diluter probes appear to be of little use in this portion of the
sampling system, although they may be useful in the downstream portion just ahead
of the particle sensor.
Any continuously operating sampling probe should be heated (unless there is
boundary layer dilution) to avoid condensation of the water vapor and acid. Heat-
ing should begin as soon as the gas gets outside the duct, and the temperature
should be maintained at a temperature above the dew point of the condensable vapors.
SUMMARY
The continuous removal of a representative sample of flue gas from a duct and
the transporting of the sample to a particle—sensing instrument involve a great
deal of engineering design problems as well as basic scientific problems. The
engineering designer faces such problems as the unknown (and perhaps unmeasureable)
turbulent flow conditions in the duct, the impaction of particles on the wall of
the tube,and the condensation of acid vapors. The scientific aspects include the
ability of a particle to stick to a wall and the probability that, once stuck, it
will be re—entrained. Agglomeration and fragmentation of particles may also be a
cause for concern with particle sizing instruments.
The available data indicates overwhelmingly that the calculation of the
magnitude of particle losses in a sampling probe is subject to a variety of random
phenomena. For stack particles, where most of the mass is contained by particles
smaller than 25 pm, the error has been estimated. Around 5 — 10% error is expected
in the removal of the sample from the main flue gas stream, due to the turbulence
and to fluctuating mean velocity. As much as 10% will impact on the wall of the tube
at bends, and Brownian and turbulent eddy diffusion will cause an additional error of
about 6%. This amounts to a total particle loss of about 30%.
ThERMO-SYSTEMS INC.
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‘ I ,
d.
00
0
S
P t
rt ’
a.
‘a.
I - I
‘a.
rt
a.
—4 m
I - ..
00
o U)
ra in
• < rt
m
Z Ph
0
Pt
a
2 p’.
fl
rt
I
It
DUCT WALL
zzzzzz
AZZZZZZZZZZZ 2
NI
FLUE GAS
HEATER
QI
a’
-------
—68—
Boundary layer diluter probes are designed to reduce the particle losses in
the sampling tube and to cool the flue gas without condensing the water and acid
vapors. Unfortunately, these probes also have particle losses. More analysis of
boundary layer dilution is necessary to completely evaluate the usefulness of
the technique.
REFERENCES
719 Badzioch, S., “Correction for Anisolcinetic Sampling of Gasborne Dust
Particles”, Journal Inst. of Fuel , V. 33, p. 106—118 (Mar 1960).
829 Badzioch, S., “Collection of Gas Borne Dust Particles by Means of
Aspirated Sampling Nozzle”, British Journal of Applied Physics , V. 10,
p. 26—32 (Jan 1959).
256 Boothroyd, R. G., “An Anemometric Isokinetic Sampling Probe for
Aerosols”, Journal of Scientific Instruments , V. 44, no. 4, p.
249—253 (1967).
133 Davies, C. N., “Deposition of Aerosols from Turbulent Flow Through
Pipes”, Proc. Royal Society , V. A289, p. 235—246 (1966).
134 Davies, C. N., “Brownian Deposition of Aerosol Particles from Turbulent
Flow Through Pipes”, Proc. Royal Society , V. A290, p. 557—562 (1966).
955 Dennis, R., Samples, W. R., Anderson, D. M., and Silverman, L.,
“Isokinetic Sampling Probes”, Industrial & Engineering Chemistry , V. 49,
no. 2, p. 294—302 (Feb 1957).
1231 Falgout, D., Walter C. McCrone Associates, Inc., Chicago, Illinois,
unpublished work (1969).
122 Grindell, D. H., “An Electrostatic Dust Monitor”, lEE Proc. , Series A
(34), V. 107, p. 353 (1960).
1060 Groutsch, E. R., “Automatic Dust Sampler”, Australasian Institute of
Mining and Metallurgy Proc. , No. 191, p. 165—189 (Sep 1959).
1130 Hemeon, W. C., and Haines, G. F., Jr., “The Magnitude of Errors in Stack
Dust Sampling”, Air Repair , V. 4, no. 3, p. 159—164 (1954).
508 Lundgren, D., and Calvert, S., “Aerosols Sampling with a Side Port
Probe”, American Industrial Hygiene Assoc. Journal , V. 28, no. 3,
p. 208—215 (May—Jun 1967).
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1156 Mitchell, C. A., and Silverman, L., “The Boundary Layer Diluter — a New
Gas and Particulate Calibration Device”, Paper presented to New England
Section of the Air Pollution Control Association at Providence, Rhode
Island (Apr 1963).
257 Narjes, L., “Dust Sampler Equipment for Quasi—Isokinetic Sampling
by Means of Novel Zero—Pressure Probes”, Chem. Age of India , V. 19,
no. 8, p. 595—603 (Aug 1968).
774 Sehmel, C. A., “Validity of Air Samples as Affected by Anisokinetic
Sampling and Deposition within the Sampling Line”, Battelle Memorial
Laboratory, Richiand, Wash., Clearinghouse No. BNWL—SA—1045 (Apr 1967).
801 Sehmel, C. A., and Schwendiman, L. C., “The Effect of Sampling Probe
Diameter on Sampling Accuracy”, in: Pacific Northwest Laboratory Annual
Report for 1967 to USAEC Division of Biology and Medicine — V. II: Physical
Sciences — Pt. 3: Atmospheric Sciences , Battelle Memorial Inst., Richiand,
Wash., Clearinghouse No. BNWL—715, Pt. 3, p. 92—95 (Oct 1968).
1111 Sebmel, G. A., “Aerosols Deposition from Turbulent Air—Stream in Vertical
Conduits”, Battelle Memorial Inst., Richiand, Wash., Clearinghouse No.
BNWL—578 (Mar 1968).
1173 Sehmel, C. A., “Particle Deposition from Turbulent Air Flow”, Journal
of Geophysical Research , V. 75, no. 9, p. 1766—1781 (Mar 1970).
1229 Vitol, V., “Theoretical Limits of Errors Due to Anisokinetic Sampling of
Particulate Matter”, APCA Journal , p. 79—84 (Feb. 1966).
1175 Wells, A. C., and Chamberlain, A. C., “Transport of Small Particles to
Vertical Surfaces”, British Journal of Applied Physics , V. 18, p. 1793—
1799 (1967).
868 Whiteley, A. B., and Reed, L. B., “The Effect of Probe Shape on the
Accuracy of Sampling Flue Gases for Dust Content”, Journal Inst. of Fuel ,
V. 32, p. 316—320 (Jul 1959).
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BETA RADIATION ATTENUATION
by: John Borgos
INTRODUCTION
When any kind of radiation passes through a medium, its intensity is reduced.
The reduction in intensity may be caused by absorption of the radiation by the
matter, by absorption of a fraction of its energy by the matter, by its reflection
or angular scattering, or by combinations of these phenomena. In the case of
absorption of the radiation, the intensity reduction through a layer of matter Is
quite easily defined by the percent of radiation that gets through. The intensity
reduction In the case where some of the energy is lost to the medium is not so
easily defined because it is often a functionof the energy of the radiation.
Angular scattering and reflection are even more complicated because the scattering
angle Is usually a function of the geometry of the measuring apparatus. In general,
absorption of radiation Is called attenuation, while reflection and angular scatter-
ing are simply referred to as scattering.
Radioactiyesp pes emit three types of
ra tsgfld .g mn&raIs.. In general, alpha particles, which are really helium nuclei,
are not very energetic; a piece of paper will stop most of them. The energy of a
beta ray (which Is a high—energy electron) depends on its radioactive source.
Many sources emit relatively low energy beta rays (i.e., less than 1 MeV), while
others emit beta rays of much higher energies. Likewise, gamma rays can have
either high or low energies, depending on the particular source. A g imvi ray is
an electromagnetic wave with characteristics identical to those of X—rays of the
same energy.
The need for instrumentation to accurately measure aerosol properties in
both atmospheric and stack gases has led to investigation of the use of these
three types of rad ation for particle measurements. By passing the radiation
through a sample o , particulate matter collected on a suitable substrate, the
properties of the articles can be evaluated by studying the scattering and
attenuation of the radiation.
‘ SInce a measure of the particulate mass is of primary interest in most
measurements, a type of radiation that is sensitive only to mass is desired.
1 Alpha radiation is not energetic enough to be useful for this, since it will not
penetrate any appreciable distance. The attenuation of gamma radiation, and
particularly low energy gamma radiation, is strongly sensitive to the atomic
number of the target material. High energy gamma radiation is not appreciably
attenuated by thin layers of particles, and would therefore be too insensitive,
as well as too dangerous. The attenuation of beta radiation depends only
slightly upon the atomic number of the target material, while the scattering of
beta radiation is nearly proportional to the atomic number. 1085
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Because of the limitations listed above, alpha and gamne radiation have
not been used for monitoring particle mass concentration. Beta radiation, on
the other hand, can be made to sense particle mass accurately if a given
physical configuration is calibrated properly.
ATTENUATION AND SCATTERING OF BETA RADIATION
A radioactive isotope that emits beta radiation may emit g na and/or alpha
radiation also. The discussion considers only those isotopes that emit beta
radiation alone and which decay to a stable element. Otherwise, the alpha or
g ma radiation or the radiation from an unstable daughter product would lead to
complications that are difficult to deal. with. Beta particles emitted from the
nuclei of such isotopes possess an energy spectrum of the same general form as
that for K 4 ° (see Figure 1) • Each particular isotope has its own unique maximum
energy (E ), which is the maximum energy a beta particle may have if it is
radiated that isotope. The spectrum is continuous, and the mean energy is
roughly one—third Emax for most known radioisotopes.
The attenuation of beta radiation involves the exchange of energy between
the beta particles and the electrons in the target material. A beta particle is
surrounded by its electric fields associated with the orbital electrons of the
atoms in the matter. Sometimes the interaction results in the exchange of enough
energy to release the orbital electron from the atom, a process called ionization.
Usually the energy transferred is less than that required for ionization, however.
The magnitude of energy transfer is a complicated function of the closeness of the
collision and the energy of the beta particle. For the range of beta particle
energies of interest, it can be said generally that as the energy possessed by
the beta particle decreases, the energy exchange increases. The main reason for
this is that as the beta particle energy decreases, its velocity decreases. Thus,
the length of time for energy exchange with a particular orbital electron increases
and hence the total energy exchange increases. Lapp 1208 (p. 177) has dealt with
this in more detail.
‘ Ii
z
Fig. 1. Beta particle energy spectrum for K 40 .
Mog
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Because beta particles are nearly massless compared to itoms, they are
deflected by nearly every collision and, therefore, follow extremely crooked
paths in passing through matter. The angle of deflection for each collision
is random, as is the energy exchanged, so that any estimate of the number of
collisions a beta particle may have before being stopped must be found statisti-
cally. For this reason, an estimate of the range of a given particle in a given
absorber is subject to statistical error. For -example, one be-ta particle may be
stopped after traveling 10 cm. through an absorber, while another with the same
initial energy may be stopped after traveling only 5 cm. through the same absorber.
The ranges of individual beta particles from a given emitter through a given
absorber is even more random because each of the particles has a different initial
energy. For this reason, a theory has not been produced that explains all the
phenomena observed. Rather, empirical formulas have been developed which predict
the range of a particle, of known energy in a -given absorber. Figures 2 and 3
illustrate the range—energy relationship for beta particles of given energies in
an aluminum-absorber. For relatively high energy particles (i.e., greater than 1 MeV),
I . ; ;,
,
1
1’
I
JL’
: :L :
I
: :
: : :J
:
iItki
r
..:i:
J_ . .L.LLL_. - .
—--F•r- L LL
TTTT ’ 11T ”Li FLflir-H
2.0 - 3.0 4.0
tnerqy. (Mev) -
Fig. 2. Range of beta particle in aluminum
th e av,er4e range of beta particles is linearly proportional to the energy of the
- ‘partic1es. For lower energies this is not true, the -reason being that ionization 2
losses become significant. It should be noted that the range is expressed in gm/cm
rather than number of collisions. This is possible because the energy loss of a
beta particle passing through a layer of mass depends on the number of electrons
per unit area in this layer. Since the number of electrons is related to the mass
per unit area for agiven absorber, the range can be expressed in terms of mass per
unit area. , -
0
N
F
w
C
0
0.5
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1.0
H J jH?IH
!flE!!
T
:j J r4
2
>‘o.l
w
Fig. 3. Range of low—energy beta particles in aluminum
Strictly speaking, these curves are not applicable for any absorber
material other than aluminum because there is some dependence on atomic number
which can hopefully, be neglected in practice with insignificant loss of accuracy.
In addition, there is a dependence on the ratio of atomic number to atomic
weight (which is between 0.4 and 0.5 for all elements other than hydrogen). This
directly affects the conversion from electron density to mass density. Nader and
Ailen 55 have compared the absorption characteristics of aluminum and membrane
tape, and found them to be significantly different as shown in Figure 4. On the
other hand, Dresia, et al, 335 give data on a variety of absorbers which are in
particle form,
Fig. 4. Beta radiation absorption curves for microweb tape and
aluminum. 555
i—•I J 1.—I —‘‘••—F• •
0.1 10
Range
10 tOO 500
(mg/cm 2 of Al)
kI
I,
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and the data indicates that these absorbers have very similar absorption
characteristics as shown in Figure 5. The error on the data is given as ± 1OZ.
Thus, it appears there is room for more conclusive data for comparison of the
absorption characteristics of various materials.
k JO JO *0
Azea density
Fig. 5. Calibration curve of the Dresia beta dust meter
for different dusts. 335
One other uncertainty that needs to be considered in practice Is the effect
of the geometry of the measuring apparatus. Nader and Allen give information
that seems to illustrate differences in absorption curves for several variations
of the source—absorber distance, the source—detector distance, and the radiation
source diameter (Figure 6). The reason for the variation in the curves Is that
for certain geometries more of the scattered radiation is detected than for other
geometries.
The attenuation of beta radiation is used to give a measure of an absorber
thickness in terms of mass per unit area (e.g., mg/cm 2 ) in the following way. A
radiation source and a detector are placed opposite each other such that beta
particles are emitted from the source in the direction of the detector. After
observing the detected count rate (Ia) of beta particles with no absorber present,
the absorber material is placed between the source and detector. A new count rate,
I, then detected and recorded. To measure particulate mass, the particles are
depc sited on a suitable substrate, such as a membrane or fiber filter. The
attenuation of the count rate caused by the mass of particles on the substrate,
together with the deposition area and air volume from which the particles were
extracted, gives a measure of particle mass concentration.
TffERMO-SYSTEMSINC
2
•Soot
4 Flyash
Cement
Derne norm, dust
Gypsum
Opefl-burningcoal
-—
- —
—
—,
—— — —
—
- —
, ‘
—
—
—•
—
—
—
C
0n7g/tm’ 2
-------
—75—
TO OCIECtOR DISTANCE
O ADSORDEN DISTANCE
DIANE TEN
• • SOURCE
S. SOURCE
(‘SOURCE
I I
Q tQ SO 50 40 10 80
ALUMINU01 AOSOflOER tIlICI€MCSS-m /cm t
Fig. 6. Relationship of source—detector and source—absorber
distance to beta radiation absorption. 555
If the abscissa of the curve of Figure 7 is expressed in units of range (e.g.,
mg/cm 2 ), assuming the range — energy reLationship to be linear, and if the
coordinate is expressed in terms of relative intensity, then an approximate fit
can be made with the relationship
I/I = exp [ — MX),
0
(1)
where I/la is the relative intensity, X is the absorber area density in mg/cm 2 ,
and p is a constant with units of cm 2 /mg. Thus, if the cumulative energy
distr bution function is available, the measurement of the relative intensity, IIl ,
will yield the absorber area density with a small error.
I
90
SO
?0
80
MASS LOADING *IIALYUR
SOURCE (MOON I4, 0,&
40
w
I-
4
I,
z
I-
z
0
I ,
II :
30
10
I
0
CURVE
DI&IENSION$
I .IN I .)
004 1.70 0.0)
2.04 DES 0.11
0.55 02 1 023
0.04 C I I 00)
APPLIES 10 FINAL DESIGN.
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‘ - F
U
Fig. 7. Typical cumulative energy spectrum of beta particles.
Since the energy distribution function is not generally available, the exact
value of is found experimentally. Equation 1 can then be used to determine the
area density of an absorber. The value of Urn has been found to be a function of
E , the maximum energy of the beta particles. It is practically independent of
ti xproperties of the absorber. This function is of the form
p =a(E )
m max
(2)
where is expressed in mg/cm 2 , and Ema is expressed in MeV. The values presented
by different investigators vary considerhly. Table 1 gives a partial list of the
values found in the literature.
table 1. Values of the constants a and b for determining the mass
absorption coefficient as given by various investigators.
Investigator
a
b
1209
Shumilovskii & Milttsen
0.022
1.33
1205
Evans
0.017
1.14
1206
Hart
.
0.0151
1.50
1144
Gleason, Taylor, & Tabern
0.017
1.43
ENERGY
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The uncertainty in the values for a and b result from the fact that the
cumulative energy distribution curve in Figure 7 is not exactly represented by
an exponential function. Thus, the geometry of the experimental apparatus (e.g.,
the source window thickness, the distance from the source to detector, etc.),
which determines the region of the curve that is used, significantly affects the
values of these constants.
Calibration of the setup with an
to control these uncertainties. With
materials which are fairly similar to
measured.
absorber of known thickness is the only way
proper calibration, thicknesses of any
the calibration absorber material can be
The scattering of beta radiation is not nearly as straightforward as absorption
(or transmission) of beta radiation. The back—scattering of beta radiation has been
found by Dresia 1085 to vary roughly as Z 08 , where Z is the atomic number of the
absorber. Because of this dependence, beta scattering is not suited to measuring
absorber area density unless the composition of the absorber is accurately known,
which is not the case with a stack aerosol.
If, on the other hand, the area density of an absorbei can be gaged (e.g., by
beta absorption), the back—scattering or scattering at some angle can be calibrated
to give a measure of the composition of the absorber. Dresiai.U1 8 5 has successfully
used the back—scattering of beta radiation to measure the ash content of ground
coke. His data shows good sensitivity for two different radioactive sources, RaD
and Tl—204 (Figure 8).
r.
0
4
I ?.
Absolute ash content
Fig. 8. Count rate for scattered beta radiation as a 1085
function of the ash content of the ground coke.
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DUST COLLECTION TECHNIQUES
In order to use the beta radiation attenuation technique to measure particle
mass concentration in a stack, it is necessary to concentrate the particles. This
is an obvious requirement because the expected relative particle concentration is
only about iO— (i.e., the ratio of mass of particles to mass of air in a given
volume is 10—5). Since the attenuation is only dependent on the total mass, which
of course includes the mass of air, an instrument would have to be able to be
sensitive to changes of 0.001% in order to be useful at all. Such instrumentation
does not exist.
Several techniques have been tried which concentrate the particles by deposit-
ing them on a tape or similar substrate material. These are the only concentration
techniques that hold any merit at this point. A few other ideas have been suggested,
including one which prevents the need for depositing the particles by concentration
of the particles by passing the carrier gas through a nozzle and keeping the
particles in the center of the stream, but it is doubtful that any of these methods
can be made to concentrate the particles by the required 4 or 5 orders of magnitude.
A review of the four known deposition techniques follows. All four methods appear
capable of operating satisfactorily with beta attenuation sensors.
The first and most obvious means of concentrating particles is to collect
them on a filter. By varying such parameters as flow rates and sampling time,
a wide range of sensitivities can be achieved. A measurement of attenuation
of radiation by the filter both before and after particle deposition yields a
measure of the particle mass per unit area on the filter, which is a measure
of the particle mass concentration in the carrier gas. The advantage of collecting
.particles on a filter is that the flow rate need not be controlled so precisely,
and collection efficiency can be very high for a wide range of particle sizes.
A major problem is the periodic movement and replacement of the filter material.
A second method of collecting particles on a substrate is to use a modified
electrostatic precipitator in which the particles are given a unipolar charge
and deposited electrostatically on a membrane. This generally requires a conducting
membrane to drain off the charge from the particles. Aluminum foil has been
suggested for this purpose by Horn. 248 Such a system could be made automatic by
using a continuously moving membrane whose feed rate could be adjusted to achieve
maximum sensitivity under a range of loading conditions. This method would require
a significant amount of development before becoming practical.
A third method is to use an impactor, provided nearly all mass of the particles
is contained by particles large enough to be collected (i.e., >0.3ii). Lilienfeld 11 ° 7
has successfully tested this technique for collection of atmosph ric aerosols being
measured using beta attenuation. This method provides the greatest sensitivity
becau8e the particles can be deposited on such a small spot on the impaction surface,
Thus, good time resolution may be attained, on the order of seconds for concentrations
found in stacks. Some method of cleaning or replacing the impaction surface is
required.
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A fourth method is to use a cyclone separator, in which the large particles
approach the wall and fall to the bottom. The source and detector are placed
such that they detect the mass of particles collected at the bottom of the cyclone.
Dresia, et al, 335 and Jackson, et al, 705 have used this technique with some success.
The concentration of particles is even greater than with an impactor because of
high flow rates. A problem is the low collection efficiency of particles below 1
or 2 microns.
BETA RADIATION SOURCES M D DETECTORS
Several general requirements can be cited for beta radiation sources and
detectors. First, the source must be energetic enough (i.e., have a sufficiently
high E ) such that some of the beta particles get through the absorber to the
detect At the same time it must have beta particles that are slow enough so
that some get absorbed by the absorber. Secondly, enough source material should
be present so that a high count rate can be attained in the detector so as to reduce
statistical error. However, the count rate must be kept low enough so that dead—time
loss in the detector are negligible (further explanation of this type of error is
given below in the discussion on detectors). Thirdly, the source should have a
fairly long half—life so that changes with time are insignificant over a period of
at least several months. Finally, a source should not be so energetic as to
endanger the health of the operators.
With any of the particulate collection techniques described above, the thick-
ness of the collection substrate is about 1 lag/cm 2 . To reduce errors due to non-
uniformity in substrate thickness (it may vary as much as 10% from one location
to another) and at the same time maintain good sensitivity and time resolution,
the thickness of particles on the substrate must be roughly 1 — 10 mg/cm 2 . This
requires a beta radiation source with a maximum range of from 20 to 300 mg/cm 2 ,
depending on the thickness of the air gap, source window, and detector window.
Several radioisotopes are available with such maximum ranges. Carbon 14, with
a half—life of 5568 yr. and an E of 0.155 MeV, has been used in most commercial
instruments of this type so far.max Since it decays to form stable N’ 4 and is
relatively abundant, it Is probably the most attractive candidate. Other radio—
isotopes that should be considered are thallium 204 (half—life: 2.7 yrs., E
0.762 MeV), cesium 137 (half—life: 33 yr., E : 0.518 MeV), beryllium 10 max
(half—life: 2.7 x 106 yr., Ema: 0.56 MeV), promethium 147 (half—life: 2.26 yr.,
E: 0.229 MeV).
Several techniques are available for detecting beta rad4tion after it has
pas8eçi through the absorber (i.e., the substrate plus the particulate deposit).
All that is required is a count rate of the beta particles; no beta particle energy
measuremenI need be made. The four most attractive counting instruments are the
Geiger—Muller counter, the end—window proportional counter, the scintillation
counter, and the solid state radiation counter.
T}IERMO-SYSTEMS INC.
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The Geiger—Muller (GM) counter has been used almost exclusively in instruments
for measurement of particle mass concentration. 335 ’ 11 0 7 It has the advantage of
being relatively inexpensive ($50 — $100), it can operate in a variety of environ-
ments, it is quite simple, it has a long life, and it is relatively small in size.
For count rates greater than about 1000 cps, however the counting efficiency begins
to drop off sharply due to dead—time losses. Dead times for GM counters are about
50 — 3000 psec. 1207
Proportional counters have also been used for aerosol mass measurement appli—
cations. 55 They offer several advantages over GM countets in that they have much
shorter dead times — on the order of 1 psec. 1207 This allows them to measure
higher count rates accurately and thus reduce statistical errors. However, they
are somewhat temperature sensitive, and often last only about one year. In addition,
they are about 5 times as expensive as GM counters. Thus, it seems that the only
justification for using a proportional counter instead of a GM counter would be a
high count rate that the GM counter cannot measure.
A scintillation counter has been used in a dust measuring instrument in the
Soviet UnioJ 061 . These counters have roughly the same dead time.
as proportional counters (for sodium iodide crystals), and even shorter dead times
are possible. They are extremely temperature sensitive, although this may not be
important since pulse height is not important. They are also very fragile, and
have lifetimes ranging from months to years, depending on the phototube quality and
sealing characteristics. Perhaps the most negative characteristic is the cost,
which is about 5 — 10 times that of GM counters. The only real advantage a
scintillation counter has over a proportional counter is that it requires less
voltage. So far, this has not proven to be sufficient justification for using a
scintillation counter.
A solid state radiation detector has been used by Benarie 17 to sense beta
particles in a stack monitoring instrument. The counting rate of these counters is
about 100 times greater than GM counters. No window is required. The detectors
are very rugged. Solid state detectors cost about 2 — 3 times as much as GM counters.
The solid state detector appears to be the most favorable of the four available
detectors.
APPLICATION TO MASS CONCENTRATION MEASUREMENT IN STACKS
While the beta attenuation technique does not, strictly speaking, measure
particle mass, it does measure something very nearly identical to mass — namely
the electron density of the particles. Compared with other techniques which measure
different properties of the particles, it offers one of the most direct means of
obtaining a measurement in terms of mass. Only analytical balances, vibrating
spring—mass systems, and one or two other methods, measure mass more directly.
THERMO- SYSTEMS INC.
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The required instrumentation can be made relatively simple and reliable,
which are important considerations for long range use in stacks. The only
moving parts are those which are responsible for transporting the collection
substrate from the collection site to the measurement position and from the
measurement position to either a cleanIng position or disposal. Such machinery
can be made to operate for long periods of time without need of repair if proper
care is taken. All electronic equipment, from the output of the radiation detector
to the readout, can be digital, although analog conversion is possible for strip
chart or meter readout.
The technique requires sampling steam aspiration since the low concentration
of particles does not allow for enough sensitivity if measurements are made in
situ. Thus, it gives essentially a point measurement of particle mass concentra-
tion. This may or may not be an advantage, depending on the shapes of the mass
concentration and velocity profiles in the stack or duct where measurements are
being made. No commercial instrumgnt of this type has been made that will operate
at stack temperatures (approx. 350 F), but there appears to be no reason to expect
such an instrument could not be made. In either case, the problem of keeping the
vapor in the gas from condensing must be dealt with. These are engineering
problems that require careful analysis and meaningful testing programs.
Only limited stack sampling data has been taken with instruments using this
technique, and much of this data has not yet been made available. Nevertheless,
some data has been presented for several different pollutants, 335 and good
accuracy and repeatability appear to be possible. Errors in this data are given
as ± 10%. It is felt that these errors can be cut in half under well controlled
conditions. For an instrument that will run for long periods of time in a stack,
however, ± 10% error is the best estimate that can be made without further data.
This error does not include the error due to losses in the sampling probe. It
only includes errors in the instrument itself, such as varying tape thickness,
uncertain flow measurement, particle collection efficiencee less than 100%,
statistical variation in count rate of beta particles, and the inherent error in
the assumption of the Beer—Lambert law (Eq. 1).
A very recent program by Schnitzler, et al 1189 , extensively evaluated the
Konitest, two light transmission instruments, and two beta attenuation instruments
in a modern plant . fired by coal. Figure 9 shows the comparative results of
the five instruments in terms of manually measured gravimetric concentration as a
functipn of instrument reading. Table 2 indicates the results of these tests.
The jJà k lines in Figure 9 are the calibration curves obtained by drawing line8
through the data points. The lighter parallel lines on either side of the
calibration line denote extrapolations of the 95% confidence level calculated for
the 150 mg/rn 3 concentration level.
Even though the beta instruments in these tests were first—generation
instruments, they show excellent correlation with gravimetric mass measurements.
Second—generation instruments could undoubtedly correlate better.
ThERMO- SYSTEMS INC.
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TR.ANSMISSOMETER A
Instrument Reading
(Arbitrary Units)
Nm 3
C )
•1•1
14
41
14
C,
KONITEST
C)
•r4
14
41
14
C,
Ins
350
ta
TRANSMISSOM.ETER B
Nm 3
/
-
-
v - -_7L77
200
;:
—
—
777
—
n — — — —
o 5 10 15 20 25 30
Instrument Reading
(Arbitrary Units)
/,
J3U
— — — —
—
. -
— r —
300
250
200
:
50
— — — — — — 74 — —
,/
— --———
‘4 —
I; (2 IIII:
— i—--—— — — —-
h
. —
—
—
—
—
—
— —.1 a—
05 10 15 20 25 30 35 £0 5 50
Instrument Reading
(Arbitrary Units)
BETA A
Nm 3
--
05IO iS 20 25
Ins truxnent Reading
(Arbitrary Units)
U
•1•1
I -I
j
14
(B
BETA B
Instrument Reading
(Arbitrary Units)
Figure 9. Gravimetric calibration of two light transmissometers,
one Konitest, and two beta radiation attenuation
instruments in a coal—fired effluent duct from
Schnitzler et al 89 .
U
•rl
I i
.1
14
C,
C)
•1•1
14
41
0
c i
14
C,
In,
250
200
150
‘Do
Pa
0 5 10 15 20 25
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TABLE 2. Snn’ ry of experimental calibration of five
instruments by Schnitzler et a1 1189 .
seometer
Sick
Instrument
Contact—
Electrostatic
Konitest
Beta Radiation
Attenuation
Transmi
Durag
A
B
A B
Number of Measurements
312
292
312
29 43
Correlation Coefficient
0.63
0.87
0.92
0.94 0.81
Measurement Tolerance at
150 mg/Nm 3
+ 68%
+ 39%
+ 33%
+ 22% + 31%
Two manufacturers of beta attenuation mass monitors for stack monitoring are
known in the U.S.: Gelman Instrument Company and Research Appliance Corporation.
Their instruments cost between $8,000 and $10,000. Included are a sampling probe,
a sampling unit which includes the beta radiation source, and a digital readout
unit. One foreign company manufactures beta instruments for stack monitoring:
Verewa, Hans Ugowski & Co. from West Germany. Saphymo Srat from France and a
West German company, Frieseke & Hoepfner GMBR, manufacture beta instruments
for ambient air monitoring.
CONCLUSIONS
The use of beta attenuation mass monitors for measuring particle mass con-
centration in stacks appears very promising for the following reasons:
1) The property of the particles that is actually measured is something
very close to mass , so that errors involved in correlating the measured
property with mass are virtually eliminated,
2) The cost is moderate ($8,000 — $10,000), and can be made lower If
large volumes are produced.
3) The Instrument can be made quite reliable due to its relative
simplicity, and
4) The instrument has had some experience in stacks so that some progress
toward ‘its development has already been made.
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It has several disadvantages that require further study:
1) It requires a sampling probe, so it is subject to errors due to
turbulent flow conditions in a stack and ] osses within the probe,
2) It gives a point measurement rather than an integrated measurement
(this may not be a disadvantage), and
3) It may not have good enough time resolution to detect changes that
occur within a few seconds.
In spite of these disadvantages, beta attenuation devices are more accurate
than most other particle concentration monitors if a measurement of particle mass
is desired, and should be considered for monitoring particle i ass concentration
in stacks.
REFERENCES
1117 Benarie, M., and Bodin, D., “Mise au Point et Essais d’Utilisation d’un
Appareil Continu Pour la Mesure de Pouasieres par Absorption de Rayons
Beta”, Pollution Atmoepherigue , no. 35, p. 147—155 (Jul—Sep 1967).
1085 Dresia, H., “The Determination of the Ash—Content of Coal with Beta and
G mii Rays”, Brennstoff—Chemie , V. 43, no. 5, p. 149—152 (May 1962).
335 Dresia, H., Fischotter, P., and Felden, G., “Kontinuierliches Messen des
Staubegehaltes in Luft und Abgasen mit Betastrahien”, VDI—Z , V. 106,
no. 24, P. 1191—1195 (Aug 1964).
1205 Evans, R. D., The Atomic Nucleus , McGraw—Hill Book Co., Inc., N.Y.,
p. 628 (1955).
1144 Gleason, G.I., Taylor, J. D., and Tabern, D.L., “Absolute Beta Counting
at Defined Geometries”, Nucleonics , V. 8, no. 5, p. 12—21 (1951).
1206 Hart, H., “Radioactive Isotopes in Industrial Measuring Techno1ogy
VEB Verlag Technik , Berlin (1962).
248 Horn, W., “Process for Continuous Gravimetric Determination of the
Concentration of Duatlike Emissions”, Staub—Reinhalt der Luft (Engi.
Trans.) , V. 28, no. 9, p. 20—25 (Sep 1968).
1061 Izmailov, C. A., “Measuring the Gravimetric Concentration of Dust in
the Air using B—Radiation”, Zavodskaya Laboratoriya , V. 27, no. 1,
p. 40—43 (1961). English translation in: Industrial Laboratory .
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705 Jackson, M. R., Lieberman, A., Townsend, L. B., and Romanek, W.,
“Prototype Fly Ash Monitor for Municipal Incinerator Stacks”, Proc.
1970 National Incinerator Conf. , Cincinnati, Ohio, p. 182—188
(May 17—20 1970).
1207 Kaelble, E. F., ed., Handbook of X—Ray , McGraw—Hill Book Company (1967).
1208 Lapp, R. E., and Andrews, H. L., Nuclear Radiation Physics , Prentice
Hall, Inc. (1948).
1107 Lilienfeld, P., “B—Adaorption—Impactor Aerosol Mass Monitor”, GCA
Technology Division, Bedford, Mass., Presented at American Industrial
Hygiene Assoc. Conference, Detroit, Mich. (May 1970).
555 Nader, J. S., and Allen, D. R., “A Mass Loading and Radioactivity
Analyzer for Atmospheric Particulates”, APCA Annual Meeting No. 52,
Los Angeles, Calif. (Jun 1959).
1189 Schnitzler, H., Maier, 0., and Jander, K., “Messtand fur die Prufung und
Kalibrierung von Registrierenden Staub — und Gasmessgeraten in einem
Steinkohlengefeuerten Kraftwerk”, SchrReihe Ver. Wass. — Boden Lufthyg.
Berlin—Dahiem , V. 33, Stuttgart (1970).
1209 Shumilovekil, N. N., and Milttsen, L. V., Radioactive Isotopes in
Instrumentation and Control , The MacMillan Co. (1964).
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P1 EZOELECTRIC MICROBALANCE
by: John C. Olin
INTRODUCTION
Piezoelectricity is a property of certain crystals, including quartz,
which results in an electrical charge on certain surfaces of the crystal
when the crystal becomes mechanically stressed. Conversely, a piezoelectric
material becomes mechanically strained if an electrical charge is placed on
certain of its crystal faces. A piezoelectric crystal, when placed in an
appropriate electronic oscillating circuit, will cause the circuit to oscillate
at the natural vibrational frequency of the crystal.
When foreign material adheres to the surface of a vibrating piezoelectric
crystal, the natural frequency of vibration of the crystal decreases. The
magnitude of the frequency change is directly proportional to the mass of the
foreign material. Some piezoelectric materials, such as quartz, vibrate at
very precise natural frequencies, so that frequency changes of one part in
ten million are significant and easily detectable. This principle has been
used recently to measure the mass of aerosol particles deposited onto the
sensing surface by an electrostatic precipitator or an impactor. 244 ,l 222 ,l 223
Piezoelectric microbalances also have had wide commercial application for
monitoring the thi kness of sputtered or evaporated thin films.
Quartz crystals can vibrate in several different modes. Several common
vibrational modes for quartz crystals are shown in Figure 1: thickness—shear
mode, length—longitudinal mode, free—shear mode, and flexural mode. In all
modes, except the flexural mode, the resonant frequency is inversely proportional
to the characteristic length L for vibration:
(1)
where N is a constant depending on the density and elastic properties of the
crystal. The change in resonant frequency is:
df c l i i i
—= ——. 2
f in
Eq. (2) is strictly true only for relatively small mass changes (1%, or less) 244
THERMO-SYSTEMS INC.
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1. Thickness—Shear Mode
N
2. Length—Longitudinal Mode
3. Face—Shear Mode.
N
4. Flexura]. Mode
Nt..
L 2
Thickness L
f — Frequency of vibration . .
N = Frequency constant for fundamental mode
L Characteristic dimension for vibration
Fig. 1. Several types of quartz crystals.
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The type AT crystal, shown in Figure 2, which vibrates in the thickness
shear mode, is usually the most useful type for measuring the mass of foreign
material, such as small particles, because there are no vibrational nodes on
the mass—sensing surface of the crystal. Thus, the mass of all particles is
detected equally regardless of their location on the sensing surface. This is
not the case with some types of crystals, such as those which vibrate in a
flexural mode. Another advantage of the type AT crystals for most mass—sensing
applications is the flat, plate—like shape which makes instrument design easier.
Fig. 2. Type AT quartz crystal in thickness shear mode of
vibration.
The use of vibrating piezoelectric crystals for mass measurement (piezo—
electric microbalances) was discovered by Sauerbrey in 1959 •1221 Sauerbrey
showed theoretically and experimentally that vibrating type AT quartz crystals
have a linear frequency decrease in response to the mass of thin metallic films
ad4 d to the electrode surface of the crystal. The relationship between added
mass and the change in natural vibrational frequency for type AT quartz crystals
is:
AM=— A
2.27 f
0
(3)
THERMO-SYSTEMS INC.
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—89—
change in natural vibrational frequency, Hz
natural vibrational frequency of the crystal, MHz
2
A = electrode area of the crystal, cm
AM mass added to the electrode area, iigm
EquatIon (3) was developed by Sauerbrey 1221 for thin metallic films, bu the
theory holds for any other material which sticks to the electrode so well that
it vibrates with the crystal.
PRINCIPLE OF OPERATION
244,1222,1223
Recently the piezoelectric microbalance has been used as an
effective means of measuring the mass of airborne particles diposited onto the
crystal surface. Particles are deposited onto the crystal surface with any
collection device, as shown in Figure 3. At the present time, electrostatic
precipitators, as shown in Figure 4,244,1222 and the irnpactors 2444223 have
been used as particle collectors. In both of these devices, the aerosol to be
measured is drawn through the sampling head with a vacuum pump.
where:
METALLIC
ELECTRODES
FORCE FIELD
GENERATED BY
PARENT DEVICE
PIEZOELECTRIC
OSCILLATOR
CIRCUIT
Fig. 3. Schematic system for tranducing particle mass using
quartz crystal oscillator.
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A complete instrument system for particle mass concentration measurement
consists of the following components:
(1) Primary crystal used to weigh deposited particles.
(2) Particle collector. A vacuum pump draws the aerosol through
the collector.
(3) Reference crystal used to subtract out possible frequency changes
caused by changes in secondary phenomena, such as operating temperature.
(4) Oscillator circuits for both primary and reference crystals.
(5) Mixer circuit to subtract the signals from the two crystals.
(6) Digital frequency counter to monitor mixer output, f, which is
proportional to the integrated mass of the piezoelectric deposition.
(7) (Optional) Direct particle mass concentration computer for calculating
particulate mass concentration, which is proportional to df/dt.
Fig. 4. Side view of electrostatic precipitator and piezoelectric micro—
balance: 1) precipitating chamber, 1/4 in dia x 1/4 in high, 2)
corona needle, 3) needle adjusting screw, 4) teflon precipitating
block, 5) high corona region, 6) entrance nozzle to precipitating
chamber, 7) piezoelectric crystal sensor.
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The entire system, except the collector, is shown in Figure 5.
PRIMARY R FERSt1CR
C RYsTM. CUVUTA!.
r ir i
F ity I F kCrkR NCI. I
OSCILLATOR I OSCILLATOR
C1RCUI1 CIRCUIT
I”
I , I
I VRU)UCACY I MASS
I COUNTbR I IC NTKATIO
T EQtJLllLY
(to oge to Eqn. 4)
Fig. 5. Piezoelectric microbalance electronic block
diagram. The frequency counter and the direct
mass concentration indicator are optional data
readout methods.
The piezoelectric particle microbalance is used to measure the mass con-
centration C (ugm/m 3 ) of airborne particles:
1
CMA(SQEE)
where:
change in mixer frequency (Hz),
At sampling time (eec),
Q — sampled aerosol flow rate (m 3 /sec.),
S Af/M1 theoretical mass sensitivity, Eq. (1) (Hz/ igm),
E = efficiency of particle collection by the collector, and
E — efficiency of the piezoelectric microbalance in weighing
the deposited particles.
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For typical numerical values used with ambient atmospheric aerosols, first
assume a 5 MHz type AT quartz crystal with an electrode diameter of 0.635 cm.
This yields a mass sensitivity from Eq. (3), of
S — —180 Hz/pgm. (5)
Further, assuming that E and E are 100%, and Q is 1 SLPX, then, from Eq. (4)
we get,
C — 333 f. (6)
Piezoelectric microbalances with an impactor collector have been used to
measure the impact of individual particles 1 ’ 23 . The digital mixer output is
converted to an analog signal and differentiated. The resulting pulse height
is used as a measure of particle size. Particle density must be assumed. The
instrument is insensitive to particles with a size less than about one micron.
For the dense aerosols found in stacks, coincidence will probably be a problem.
For these reasons, this use of the piezoelectric microbalance is not recommended
for stack application.
DESIGN CONSIDERATIONS FOR STACK MONITORING
An ideal instrument for measuring total particle mass concentration will
have E = 100% and E = 100%. Impactor collectors can never have 100% collection
effici ncy because p rticles with a size less than the inipactor’s particle size
cut—off will escape the collector. This property of impactor collectors makes
them useful for particle size—distribution monitoring. An electrostatic pre-
cipitator collector does have the potential for collecting particles of all
sizes. Experiments have shown that the collection efficiency E of an electro-
static precipitator similar to the one shown in Figure 4 is ess ntially 100% for
several types of particles over a wide mass concentration range (approximately 10
to 10,000 iigm/m 3 ). Aerosols tested Included tobacco smoke, indoor aerosol, and
atmospheric aerosol. Figure 6 shows these tests.
A 100% weighing efficiency E requires perfect adherence of the particles
to the vibrating crystal surface. Particle adhesive force is a complex com-
bination of molecular attraction forces, electrostatic forces, and surface—
tension forces 244 . As particle size Increases, the ratio of particle adhesive
force to inertial force decreases. Hence, the mass of particles above a certain
critical size will not be weighed with 100% efficiency by the piezoelectric
microbalance. The critical size is maximized by depositing particles uniformly
onto the crystal surface and by minimizing crystal drive level. The experimental
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CORONA CURRENT, J4AMPERES
Fig. 6. Results of electrostatic precipitator efficiency tests.
data of Figure 7 shows a weighing efficiency of nearly 100% for tobacco smoke
and room aerosol. The tobacco smoke aerosols were diluted by factors of 5 to
20 with relatively clean ambient air, thus stablizing any condensation of vapors.
The aerosols used for obtaining data in Figure 7 did not have significant mass
above 10 microns.
tOO
75
6
Z50
LU
Li
t i e
LU
25
00 I 2 3 4 5 6 7 8 9 10
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Fig. 7. Experimental mass sensitivity of 1.6 MHz type AT
quartz crystal (electrode area = 1.06 cm 2 ).
The adherence of particles to the crystal surface can be increased by several
other methods. The application of a thin coat of sticky material to the crystal
surface is one methOd. This technique has been used for many years to prevent
particle bounce and blowoff from the collection plate of aerosol impactors. The
application of a thin uniform film of sticky material to the crystal electrode
may be a problem. However, various coatings have been applied to crystal surfaces
for measurement of trace gases. 1 - 352 En this case, the trace gases are selectively
adsorbed by the coating, causing an increase in the mass on the electrode which is
detected by a corresponding crystal frequency shift. Thus, thIn films of coating
material can apparently be applied to the crystal. The film of material must, of
course, remain stable during the particulate deposition process . Any mass changes
in the film material itself must be insignificant compared to the mass of deposited
particles. A sticky film on the crystal surface will not increase the adhesion of
more than one layer of particles.
N
I
KEY.
A-TO ACCO SMOKE
.
0- ROOM AEROSOL
2000
—
/
1600
—
—
—
—
—
—
—
1400
—
— ——
l-
THEORY .—
600——-
0
.
40 C V- --
-
0 50 tOO 50 200 250 300 350 400 450
AM (MICROGRAMS)
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Another way to make particles adhere more strongly to the crystal is to
coat the airborne particles with a thin film of sticky material before they are
deposited onto the crystal surface. This method has been used with piezoelectric
microbalances which sense atmospheric particle mass concentration ) 3 A trace
amount of ammonia vapor, or a similar vapor material, can be introduced into the
sampling line upstream of the crystal. Approximately one monolayer of material
adheres to the particles. This layer, in turn, attracts one or two inonolayers
of water. The resulting liquid coating contributes insignificantly to the mass
of particles, but does increase the surface—tension force causing particles to
adhere to the crystal. The water layer also decreases the electrical resistivity
of the particle, allowing electrical charge to bleed off the particles in an
electrostatic precipitation—piezoelectric instrument. This same principle has
been used in the past to improve the performance of large electrostatic collectors
used to reduce particulate emissions from smoke stacks.
If the deposited particulate mass becomes excessive, the oscillator circuit
can no longer drive the crystal stably. When this occurs, the crystal must be
wiped clean either automatically or manually. For cigarette smoke, a mass
loading of approximately 250 micrograms is achieved before encountering
instability. Since a mass of only 0.5 micrograms is over 100 times greater
than the sensor resolution, it is theoretically possible to obtain 500 readings
with tobacco smoke before cleaning the crystal. For atmospheric aerosols the
total mass loading is probably in the range of 25 — 50 micrograms. Thus, the
maximum loading depends on the type of aerosol. For stack emissions, the maximum
losding will probably fall between 25 and 50 micrograms. The necessity to clean
the crystals is an obvious disadvantage of piezoelectric particle microbalances.
However, it appears that several automatic cleaning methods could be developed.
Quartz crystals are sensitive to temperature. Type AT quartz crystals are
selected, in part, because they belong to a class of low—temperature coefficient
crystals, with values of a fraction of a part per million per°C, as shown in
Figure 8. The tyge AT crystal shown in Figure 8 has a “zero” temperature
coefficient at 20 C. By changing the orientation angle of the type AT, the zero
temperature coefficient point can be increased to about 60°C. Thus, it will be
necessary to cool the gases to that temperature, or to 0 find a different crystal
type which has a zero temperature coefficient near 150 C.
Piezoelectric particle microbalances make a point measurement. Hence, they
require extracting the sample from the aerosol stream. Like other such instruments,
they are possibly subject to sampling errors and to particle loss, vapor condensa-
tion, and particle agglomeration in the sampling train. Agglomeration does not
affect the total mass concentration measurement, but does affect size—distribution
monitoring. If water condenses on the crystal, erroneous results, and even in-
stability, can be encountered. Diluting with clean dry air should eliminate
the problem. Alternatively, the entire sampling train and sensing head could be
operated at the stack temperature (nominally 150°C) to prevent condensation.
ThERMO-SYSTEMS INC
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I
Fig. 8. Temperature frequency characteristics of zero
temperature coefficient quartz crystals.
Crystals with a 150°C zero temperature coefficient, for example, type BT, would
be used in this case. In any case, the condensation of several vapors in the
effluent must be dealt with carefully in the design of a piezoelectric sampler,
or in the design of any other extraction sampler.
APPLICATION TO PARTICLE MASS CONCENTRATION MONITORING IN STACKS
Table 1 gives the properties of typical quartz crystals vibrating in the
four modes shown in Figure 1. The length—longitudinal and flexural modes have
greater damping than the two shear modes: thickness shear and face shear.. In
face—shear mode, the’particle layer is strained, an undesirable feature. For
these reasons, the thickness shear mode is most desirable.
Types AT and BT quartz crystals have the best temperature coefficient
characteristics of crystal vibrators in the thickness—shear mode:
(a) The temperature coefficient is very low.
(b) The temperature coefficient is relatively flat over a reasonable
temperature range.
(c) The range of flat temperature coefficient can be increased (with BT)
to cover the high temperature stack environment.
If necessary, low frequency (approx. 500 KHz) type AT crystals can be used for
stacks if the sensing of large particles is a problem.
Although types AT and BT crystals appear to be best for stack monitoring,
crystals vibrating in the length—longitudinal mode may be used if very low
frequencies are necessary to sense large particles.
TtMPCRATURC Ill DCGREU C
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TABLE 1. PARAMETERS FOR TYPICAL PIEZOELECTRIC CRYSTALS
App rox. Frequency
Constant N for A.pprox. Temperature Co—
Frequency Fundamental Mode efficient (garts per
Type Shape Range(KHz) (lUtz mm ) million per C)
Thickness— AT Plate 500 to 15,000 1,660 0.0 (vary angle of cut
She ar to achieve 0.0 from 20 C
to 100°C)
ST Plate 1,000 to 2,560 0.0 at 25°C (not as
20,000 flat as AT)
I PLate 500 to 20,000 1,981 +75 to 125
AC Plate 1,000 to 1,656 +20
15,000
BC Plate 1,000 to 2,611 —20
20 ,000
Lengt.Ir
Longitudinal
X
Rod
40
to
350
2,870 (varies with
size ratio)
- 0
MT
Plate
50
to
500
2,700 (varies with
size ratio)
0
Face--Shear
CT
Plate
300
to
1,100
3,070
0.0
at
25°C
DT
Plate
80
to
300
2,060
- 0
Flexural
NT
Rød
4
to
70
283 (for width to
length ratio=O.05)
+ 0
Thus, the optimum crystal type and frequency for stack application may be
different than the crystal used for monitoring atmospheric aerosols. liowever, to
estimate the essential performance of a piezoelectric microbalance for stack
application, let us assume that the crystal required is the same as that used for
other aerosols. We will impose the further criterion for a good total mass con—
cu3ntration instrument that all of the particles are collected on the crystal
surface, i.e., B 100%. This implies the use of an electrostatic precipitator
to deposit partiE les onto the crystal surface. However, an impactor may be
applicable if the particles are relatively large, e.g., if 90% of the mass is
above 1 micron. The weighing efficiency Ew is also taken as 100%. With these
assumptions, anticipated instrument performance in stacks is sumaarized in Table 2.
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—98—
TABLE 2. TYPICAL PERFORMANCE CHARACTERISTICS
1. Crystal frequency
2. Crystal diameter
3. Sampled gas flow rate
4. Mass Sensitivity S (Eq. 5)
5. Frequency resoiution
6. Mass resolution
7. Time resolution (Eq. 6)
3 (b)
a. After collector — high value, C = 3 guts/rn
3 (b)
b. After collector — low value, C = 0.03 gins/rn
8. Approximate theoretical number of measurements before
cleaning crystal(c)
9. Time before cleaning assuming continuous sarnpling
a. After collector — high value, C = 3 gms/rn
Notes:
3 (b)
b. After collector — low value, C 0.03 guts/rn
5 MHz
0.635 cm
1 SLPM
— 180 Hz/ugzn
± 1 Hz approx.
± 0.0056 ugm
0.098 sec.
9.8 sec.
100
0.16 m m.
16 mm.
(a) Experiments show that frequency resolution is limited by the
frequency counter (± 0.1 Hz), rather than by the piezoelectric
microbalance. Nevertheless, we have conservatively chosen
a ± 1 Hz frequency resolution.
(b) This time resolution is based on the collector of enough mass so that
the specified frequency reso1u tion is 1% of the frequency shift caused
by the mass. A pulverized coal boiler is assumed. The mass concentra-
tions assumed represent the typical range expected in coal—fired stacks.
The collection and sensing efficiences are assumed to be 100%.
(c) Assuming a total frequency shift of 10 KHz before encountering in-
stability, ± 1 Hz resolution, and 100 Hz per measurement.
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—99—
A major disadvantage is that the crystal must be cleaned relatively
frequently. This disadvantage can be dealt with in several ways:
(1) Operating the electrostatic precipitator only when a measurement
is desired — for a few seconds at a time.
(2) Providing a means of automatically cleaning the crystal or indexing
another clean one into the precipitator chamber.
(3) Diluting the sampled aerosol to reduce the particle concentration.
(4) Using crystals with a higher total frequency shift, such as lower—
frequency, type AT crystals.
All four methods could probably be incorporated into an instnm ent.
The low sampling rate (about 1 LPM) of piezoelectric monitors may require
a special sampling probe system to prevent excessive particle losses. Figure 9
shows one possible system. A separate system removes a convenient, larger flow
from the stack and slows it down so that a reasonably—sized probe can remove
a sample at the low flow rate necessary for the piezoelectric monitor. The sample
may be conditioned by dilution or sample tube heating or cooling, if necessary.
1k
V I 1
FLeE
1 ___
Fig. 9. A possible stack sampling system using a piezoelectric
microbalance sensor.
PUMP
VALVE
PUMP
;AMPL
C0 ‘OITIOIER
FLCWMCTER
APPROXIMATELY
150 LITER iMINUTE
APPROXIMATELY
I LITER/MINUTE
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Another potential problem is the weighing of larger particles, as discussed
earlier. Fortunately, downstream from electrostatic collectors, approximately *
95% of the particulate mass Is less than 25 microns for all coal—fired facilities.
Experiments with atmospheric aerosols show that the upper size cut—off with the
piezoelectric mlcrobalance described in Table 2 is greater than 10 — 1 microns.
The exact size cut—off is not known.
If particles In stacks are sticky, they should adhere firmly to the oscillat—
lug crystal. This would increase the upper size cut—off. Lower frequency type
AT crystals, such as 1.5 MHz, also can be used to increase the upper size cut—off.
Several methods discussed earlier can be used to increase the ability of the micro—
balance to sense large particles. Thus, the piezoelectric microbalance has good
promise for weighing particles up to 25 or more microns in coal—fired sources.
Piezoelectric microbalances appear to be quite adaptable to monitoring
emissions from oil—fired facilities. Particle sizes are typically small (less
than approximately 1 micron). The particles are sticky. Mass concentrations are
about 10 times less than coal—fired emissions, resulting in 10 times more instru-
ment operating time before cleaning Is required.
As mentioned previously, water or vapor condensation on the crystal can cause
erratic operation. This can probably be overcome by diluting the sampled gas and/or
heating the crystals.
CONCLUSIONS
The piezoelectric particle microbalance for measuring the total mass con-
centration of particulate emissions from stationary combustion sources has the
following advantages:
(1) It measures particle mass only; it is thus an instrument universally
applicable to all stacks without calibration.
(2) It is very sensitive, resulting in high accuracy and time resolution.
(3) It is simple, compact, and rugged, which leads to reliability and
moderate cost ($7,000 — $10,000).
(4) It can be easily used with several particle collectors, including
impactors and electrostatic precipitators.
It has the following disadvantages:
(1) The crystals require cleaning. If done automatically, this
represents an additional system complexity.
*See properties of emissions elsewhere in this report.
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(2) A potential upper particle size limitation exists.
(3) The instrument makes a point measurement and, with the small
sample rate allowable, this may be a disadvantage.
(4) An extracted sample is required. Without dilution or heating this
will result in water or other liquids condensing on the crystal
and subsequent unstable operation.
(5) The instrument has not yet been used in stacks.
The disadvantages do not seem prohibitive. They can probably be overcome
with the appropriate designs discussed in the text.
REFERENCES
1223 Chuan, R. L., “An Instrument for the Direct Measurement of Particulate
Aerosol Science , V. 1, p. 111—114 (1970).
1352 King, W. H., Jr., “Using Quartz Crystals as Sorption Detectors, Parts
1 and 2”,Research/Development , V. 20, no. 28, p. 28—34 (Apr 1969), and
V. 20, no. 28, p. 28—33 (May 1969).
244 Olin, J. G., and Sam, C. J., “Piezoelectric Aerosol Mass Concentration
Monitor”, paper presented at Syinp. on Advances in Instrumentation for
Air Pollution Control, Cincinnati, Ohio, May 26—28, 1969.
1222 Olin, J. C., Sam, C. J., and Christenson, D. L. “Piezoelectric/Electro—
static Aerosol Mass Concentration Monitor”, Presented at Amer. md. Hyg.
Assoc. Annual Conference, Detroit, Mich., (May 11 — 15, 1970).
1351 Olin, 3. G., Trautner, R. P., and Sent, G. J. “Air—Quality Monitoring of
Particle Mass Concentration with A Piezoelectric Particle Microbalance”,
Paper No. 71—1, 64th Annual Meeting, Air Pollution Control Association,
Atlantic City, N.J. (Jun — Jul 1971).
1221 Sauerbrey, G. A., “Verwendung von Schwing—quarzen sur Wagund dunner
Schichten und zur Mikrowagung”, Seits. Phys. , V. 155, p. 206 (1959).
REFERENCES NOT REFERRED TO IN TE
1187 Leemhorst, J. W., “Direct Measurement of Particulate Mass and Humidity”,
Contamination Control , p. 11—14 (Jul—Aug 1970).
252 Rogallo, V. L., and Newman, F., “A Wide—Range Piezoelectric Momentum
Transducer for Measuring Micrometeoroid Impacts”, Ames Research Center,
Moffett Field, Calif., Clearinghouse No. NASA TN D—2938 (Jul 1965).
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RESONANT FREQUENCY
by: 3. G. Olin and F. D. Dorman
INTRODUCTION AND PRINCIPLE OF OPERATION
The resonant, or natural, frequency of a vibrating spring—mass system
decreases if the mass is increased. The same is true of any mechanical system
vibrating at its resonant frequency.
Figure 1 shows some typical vibrating mechanical systems — the spring—mass
system, the torsional system, the cantilevered beam system and the vibrating wire.
In all cases the resonant frequency f is inversely proportional to the square root
of mass m, or
f c__, (1)
&
Where C is a constant depending on the geometry of the vibrating system and its
elastic, inertial, and force constants. The change in resonant frequency df is
— 1/2 • , (2)
Figure 2 shows one possible mechanical resonant frequency system. The
physical configuration is tubular with Figure 2 being an axial cross—section. The
filter is made of rigid, porous material while the thin—walled tube can flex to
some degree. The oscillator circuit—magnetic coupling causes the filter—tube
system to vibrate torsionally at its natural frequency. As particles collect in
the filter, the natural frequency decreases in proportion to the mass of added
particles. A frequency counter monitors the output. The filter may be sintered
metal or porous ceramic. The filter could possibly be cleaned periodically by;a
reverse blast of air or liquid cleaning solution.
Piezoelectric crystals, such as quartz crystals,constitute a class of resonant
frequency devices. They are discussed in detail in the section entitled “Piezo
electric Microbalance t ’.
APPLICATION TO MASS CONCENTRATION MEASUREMENT IN STACKS
Mechanical Vibrators
Since nearly all mechanical resonant frequency devices, including the one
described in Figure 2, are only in the conceptual stage, no data or specific
working hardware can be discussed. The remainder of this discussion is limited to
general concepts.
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1. Spring—Mass System
ir
f = —
21T Ym
kr.Spring Constant
i n Mass
ar tides
2. Torsional System
3. Cantilever System
r
f= • .;— ti
V mL
4. VibratIng Wire
i
stiffness
Fig. 1.. Typical mechanical vibration systems.
J Polar moment
of inertia
K —Torsional
Constant
T1IERMO-SYSTEMS INC.
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- —lO4 -
Particles can be deposited onto resonant frequency devices in several ways,
including the following coon methods:
(a) Impaction,
(b) Electrostatic precipitation, and
(c) Filtration.
In mechanical systems it is advantageous to deposit the particles onto
a non—vibrating surface which is unstrained, such as a large”dead” mass. Other-
wise, the particulate layer will become strained. Since Its elastic properties
are undoubtedly different than that of the vibrator, they become additional
variables which must be taken into account. This should be avoided. None of
the systems shown in Figure 1 have this problem.
The use of resonant mechanical systems for particle mass measurement is a
very new field. The work of Cast’ 228 with the vibrating wire shown in Figure 1
is the only reported reference known by the authors. Cast uses a relatively
complicated feedback control system to maintain the wire at constant tension.
The output of the system Is the voltage necessary to maintain constant tension.
As with piezoelectric particle microbalances, a reference wire which is kept
free of particles is used to subtract extraneous changes in temperature, etc.,
out of the signal. The major problem with the vibrating wire is the difficulty
of distributing a representative particulate sample on the wire. Studies of
the collection efficiency of fiber filters show that suspending a wire across
the stack simply will not have a high or repeatable collection efficiency.
Unless a good particle collection mechanism is developed, the vibrating wire is
not a good candidate for stack monitoring. It is doubtful that such a collector
can be devised.
Shear (e.g., torsional) vibrating systems are generally superior to long-
itudinal or transverse (spring—mass, centilever, and vibrating wire) vibrating
systems. The major energy loss is the viscous loss caused by the shear stress
in the hydrodynamic boundary layer. On the other hand, longitudinal or trans-
verse systems alternately compress and expand the adjacent fluid (air) and I
create sound waves. This requires relatively large amounts of energy to over-
come the resulting damping.
In the ideal case of no damping, any vibrating system has zero energy loss.
Th amplitude of such a vibrating system is constant. This means the system
has infinite sensitivity, i.e., it will vibrate at precisely the, resonant fre-
quency of the system. As the damping of the system Incveases, the resonant
amplitude decreases with an accompanying broadening of the amplitude—frequency
peak, resulting in loss of sensitivity. This phenomenon is shown in Figure 3
for the damped spring—mass system 1239 . In electronics, the parameter Q is a
measure of the sharpness of the amplitude—frequency distribution. A very high Q
is desirable for generating precise frequencies. For these reasons, shear systems
are preferable to other systems. The same conclusion Is also true for piezo—
electric systems.
ThERMO .SYSTEMS INC.
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—105—
Based on these considerations, a torsional system appears to be the best
mechanical system. A torsional system with good potential for stack monitoring
is shown in Figure 2. Particles are captured with a filter. This permits
collecting essentially all particles with 1002 efficiency. The filter must be
cleaned or replaced periodically. It could be cleaned automatically by flushing
with a liquid. An impaction or electrostatic precipitation particle collector
could also be used to deposit particles onto the surface of the torsional system.
In this case, larger particles may not be sensed with 100% efficiency because
they may slide relative to the sensor surface.
Particles
Fig. 2. Torsional filter system transducer.
Output
Mechanical vibrating systems have a major advantage over piezoelectric
systems: they have significantly lower resonant frequencies. Therefore, they
should be able to sense larger particles because the inertial force acting
on the particles is proportional to f 2 . Lower frequencies are also associated
with reduced mass sensitivity. In stacks, this may not be a problem because
particle concentrations are high. Hence, a large mass of particles can be
deposited onto the sensor in relatively short time periods.
A reference vibrator, identical to the primary vibrator used to measure
particle mass, can be used to subtract out the effects of changes in the temp-
erature of the gas stream. A mixer circuit is used to subtract the frequency
signals of the primary and reference vibrators. Furthermore, materials with
low sensitivity to temperature can be selected for the vibrating member of the
system (e.g., the tube of the torsional system). This is equivalent to the
selection of quartz crystals with low temperature coefficients. This means
mechanical vibrators can be made relatively insensitive to all environmental
and particle parameters except particle mass. This is indeed an important
advantage.
Direction of
To Vacuum Motion
tsr
Wall
Tube With
Low Temperature
Coefficient
k Magnetic
\,,Coup1in
Frequency
TRANSDUCER
THERMO-SYSTEMS INC.
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LUO
I..
C,
z
I
Fig. 3. Resonant amplitude as a function of resonant
frequency for a damped spring-mass, system from
Thomson 3 .
CONCLUSIONS
Mechanical resonant frequency systems have the following advantages for
monitoring particle mass concentration in stacks:
(a) They measure particle mass . Extraneous effects can be eliminated.
(b) They are inherently simple, rugged devices.
The main disadvantage is their infant state of development. A torsional vibrator
with a filter collector has good potential for stack monitoring. The development
of mechanical resonant frequency devices should be pursued further.
Piezoelectric vibrators constitute a class of resonant frequency devices.
They are discussed in detail in the section entitled “Piezoelectric Microbalance”.
They have much greater mass sensitivity and resolution than most mechanical
vibrators.
REFERENCES
1228 Gast, T., “Akustische Ruckkopplung als Hilfsmittel bei der
Bestimmung von Staubkonzentrationen mit Hilfe eines Schwingenden
Bandes”, Staub—Reinhalt der Luft , V. 30, no. 6, p. 235—238 (Jun 1970).
1239 Thomson, W. T., Mechanical Vibrations , 2nd Ed., Prentice—Hall, Inc.,
Englewood Cliffs, N.J. (1953).
FREQUEN f RATIO
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GRAVIMETRIC WEIGHING
by: John Borgos
INTRODUCTION
Gravimetric weighing devices have been important tools in science and
technology for hundreds of years. They are simple to understand, relatively
inexpensive to construct, easy to calibrate, and readily applied to a wide
variety of situations where a measurement of mass is required. The principle
employed is that the force due to gravity acting on an object is proportional
to the mass of the object. Since the force of gravity is familiar and fairly
constant, it is used often to define standard units of mass or weight.
A gravimetric weighing device can be used to measure the mass concentration
of particles in an airstream. The particles must be deposited on a suitable
substrate before they can be weighed. This usually involves use of an impactor,
filter, or electrostatic precipitator. The measured mass of particles divided
by the volume of air sampled gives mass concentration directly.
PRINCIPLES OF OPERATION
A gravimetric weighing device consists basically of a pivot point about
which two equal and opposing torques act. One torque is set up by the weight
of the particle sample whose mass is to be determined. The other torque is
usually controlled, either manually or automatically, such that a “null” position
is reached where both torques are equal. Several techniques are possible for
measuring and controlling this opposing torque.
The first and most obvious technique is to maintain balance by using opposing
weights (Figure 1). This instrument is traditionally known as the analytical
balance. Its operation is familiar enough that it need not be discussed further.
Such instruments can be made sensitive to mass changes of a few micrograms.
243
A second technique, described by Gast and shown in Figure 2, uses an
electromagnet to produce the restoring torque necessary to maintain zero offset.
When weight is applied to the balance beam, it tends to rotate slightly. This
rotation is immediately opposed by the electromagnet, which applies a compensating
torque around the pivot point. By measuring the current through the electromagnet
required to maintain the balance at the null position, the weight on the balance
arm can be calculated. This instrument is sensitive to a minimum weight change of
about 10 micrograms. Mauer’° 96 describes a similar instrument, but his sensitivity
is on the order of 10 milligrams.
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Pivot
Fig. 1. Basic configuration of analytical balance.
Fig. 2. Principle of the recording dust balance:
(a) suction pump; (b) separation chamber; (c) discharge
wire; (d) electrode: (a) pressure plate; (f) balance
beam; (g) microbalance; (h) control disk; (i) velvet
strip; (k) scriber; (1) wax paper roll; (m) auxiliary
blower; (ii) paper filter. 225 , 243
Sample
Balance
14us
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A third technique, described by Peterson, 938 is really a modification
that can be added to any conventional analy.t ical balance to increase its
sensitivity. The operational principle involved is that the induced voltage
in the secondary coil of a transformer is linearly related to the position of
the core for small (i.e., <.030 inch) displacements of the core. A differential
transformer is used, which has three coils: one primary and two secondary coils.
The secondary coils are electrically connected such that their output voltages
oppose each other. The core is attached to the analytical balance beam and can
move freely in the cylindrical space inside the coils. At the null position, the
core induces equal voltages in the two secondary coils, and the resultant output
voltage is zero. Any displacement of the core causes an increase in one secondary
coil voltage and a decrease in the other, resulting in an output voltage. This
output voltage can be amplified and interpreted in terms of mass loading.
Sensitivities as low as 0.1 microgram can be achieved with this technique.
A fourth technique is described by Caule and McCully 1097 for recording rapid
changes in weight (Figure 3). This, too, is really a modification that can be
made to an existing balance to increase its sensitivity. Two coils are used. One
is fixed in position on the base of the balance and is supplied with A.C. power.
The other is hung from the beam of the balance. By Lenz’s law, the primary and
secondary currents will be repulsive. Thus, by measuring the primary current
required to maintain the null position, the weight of a sample can be calculated.
Mass sensitivity is on the order of one milligram.
COPPER
PRIMARY COIL
Fig. 3. Sketch showing arrangement of coils and support
from balance arm on Cattle and McCully sensing
technique .109 7
SALANCE
COUNTER
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A fifth technique appears possible which uses a resistor whose resistance
value changes as the flux density of a surrounding magnetic field. This
phenomena is known as magnetoresistance. Such a resistor might be attached to
the balance arm of some device, and a permanent magnet could be attached such
that its magnetic field surrounds the resistor. Then any offset of the balance
arm would change the position of the resistor in the magnetic field, therefore
changing the local flux density and hence the resistance. Sensitivities as low
as one microgram might be possible.
When gravimetric weighing devices are used for measuring particle con-
centrations, a means of collecting the particles is needed, Of the instruments
described above, only that of Gast has been used for measuring aerosol mass
concentration (Figure 2). This instrument electrostatically deposits the
particles on a precipitation plate which is attached to the beam of the electro-
magnetically controlled balance. The surface on which the particles are actually
deposited is a foi L section which is automatically cut from a supply roll and
discarded after the measurement is conipleted. 225 An earlier version 243 used
a brush to clean the deposition surface, but this did not work for fly ash
because the particles were too sticky to be removed. The aspirated gas f low 3
rate is about 60 liters per minute. At a particle concentration of 100 mg/rn ,
the instrument need only sample a few minutes to get enough mass accumulated
for an accurate reading.
Either an impactor or filter would also be feasible for separation of the
particles from the airstream. In any case, the collection surface must be
locked while the particles are being collected and freed when weighing is done.
The air flow must also be turned off or diverted when weighing is done to avoid
disturbing the balance. Thus, any automatic gravimetric device must have a
mechanism for cleaning or replacing the collection surface and executing the
measurement cycle.
APPLICATION TO MASS CONCENTRATION MEASUREMENT IN STACKS
Only one gravimetric weighing device has been tested in a stack. This is
the instrument described by Gast 2 3 and later revised. 225 This work has shown 1
that the gravimetric weighing technique can be used for stack monitoring, but it
has several limitations. It must be maintained at a high enough temperature to
avoid condenstaion of vapors. It has a sensitivty limitation because it must be
rugged enough to be immune to the vibration around a stack; with this in mind,
the best sensitivity that can be achieved is around 1 — 10 mg, which represents
a fairly large mass of particles. A sampling train is required ‘in which some
particles will inevitably be lost. A sophisticated mechanism would be necessary
to clean or replace the particle collecting surface; this reduces the reliability
of the instrument and makes it only quasi—continuous. It probably would sell
for more than $10,000 when commercially developed.
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Nevertheless, the gravimetric weighing technique does possess the enviable
- advantage that it measures the dust mass and not some other physical quantity.
Therefore, since the difficulties are mainly engineering problems that do not
make the technique inherently incapable of monitoring stack gases, we place
emphasis on the value of such instrumentation. Because it is not now available
commercially, development should be postponed until several other sensing methods
are tested and proven. Many of the limitations listed above can be eliminated
in future development programs.
CONCLUSIONS
The gravimetric weighing technique appears to be a somewhat promising method
for monitoring particle mass concentration in stacks because:
1) It has the advantage that it measures the particle mass directly,
2) At least one instrument has been developed, although it is not
commercially available, and
3) The technique Is well understood, so that the problems are mainly
engineering problems.
Several disadvantages limit its immediate usefulness and make it less promising
than several other techniques:
1) It requires some sophisticated machinery, so reliability may be poor,
2) It is especially sensitive to mechanical vibrations, which can be severe
near stacks,
3) A sampling probe is required, so it is subject to errors due to
losses within the probe, and
4) It gives a point rather than an integrated measurement (further
study may well indicate this is not a disadvantage for some
applications).
Cravimetric weighing devices are inherently quite accurate for measuring
particle mass, and development work should be done in the future to fully evaluate
the usefuliness of automation. Although they are potentially a strong competitor
for stack monitoring instrumentation, immediate attention should be given to other
more veil developed methods which appear simpler and which may be used in stacks
with less additional development.
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REFERENCES
938 Aronson, M. H., and Peterson, A. II., “A New Recording Microbalance”,
Instruments & Automation , (Jul 1955).
1097 Caule, E. J., and McCul].y, G., “An Automatic Recording Analytical
Balance”, Canadian Journal of Technology , V. 33, p. 1 (1955).
225 Duwel, L., “Latest State of Development of Control Instruments for
the Continuous Monitoring of Dust Emissions”, Staub—Reinhalt der Luft
( Engl. Trans.) , V. 28, no. 3, p. 42—53 (Mar 1968).
243 Cast, T., “Staubmessgerate mit massenproportionaler Anzeige oder
Registrierung”, Staub—Reinhalt der Luft , V. 20, no. 8, p. 266—272
(Aug 1960).
1096 Mauer, F. A., “An Analytical Balance for Recording Rapid Changes in
Weight”, The Review of Scientific Instruments , V. 25, p. 598—602 (1954).
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ELECTROSTATIC MEASUREMENT METHODS
by: Gilmore J. Sem
Three basic electrostatic particle concentration measuring techniques are
available for stack monitoring:
1. ION CURRENT ATTENUATION
2. ION CAPTURE
3. CONTACT CHARGE TRANSFER
Each technique is discussed separately below. A conclusion section at the end of
this electrostatic report summarizes the potential of such techniques for particle
mass emissions measurement from large coal— and oil—fired combustion sources.
ION CURRENT ATTENUATION
As particles pass through a cloud of ions, the particles acquire an electro-
static charge which they carry away, reducing the concentration of ions in the
cloud. Assuming a constant particle size distribution, the greater the concentra-
tion of particles passing through a cloud, the greater the reduction, or attenuation
of the ion concentration. Thus, the decrease in ion concentration is an indication
of the concentration of particles. The specific concentration parameter measured
by this method will be discussed later.
Hasenclever and Siegmann 242 and Coenen 1040 ’ 245, 284 discuss one instrument
design utilizing ion current attenuation. Figure 1, taken from a review article
by Schutz 659 describes the instrument. A pump draws dusty air laminarly through
the ionization chamber. A radioactive source, 8 microcuries of Cobalt—60, is
uniformly distributed around the inner side of the outer tube, creating a low
bipolar concentration of ions. Ions of one polarity are drawn across the chamber
to the central electrode by a small electric potential. The ion current, collected
on the center electrode and measured by an electrometer, decreases with increasing
dust concentration.
lto4
Fig. 1. An ion current attenuation design from Schutz 659
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Mohnen and Holtz 58 ’ describe an instrument which uses two chambers similar
to the one in Figure 1. One chamber operates as described above with aerosol.
passing through. The second chamber operates identically except that the enter—
Ing air is filtered. This sys tem automatically compares the attenuated- current
in the aerosol chamber with the reference current in the identiéal clean—air
chaqther so that the electrical output signal is directly proportional to ion
current attenuation. In the one—chamber design, this comparison must be done
manually, a procedure which becomes quite difficult with rapidly fluctuating
aerosols. An additional advantage of the two—chamber system is that the clean—
air chamber uses the same high voltage power supply so that any voltage fluctu-
ations are nulled out.
The essential components of an ion current attenuation instrument are a
constant supply of unipolar ions and a method of measuring the decrease in ion
current between the electrodes due to ions carried away by particles. The
specific instruments described above use a radioactive ion sourc e because it
supplies an essentially constant supply of ions. Since the radioactive source
produces ions of both polarities, an electric field across the flow channel is
necessary to eliminate one polarity. Fluctuations in the voltage producing the
electric field result in similar fluctuations in measured ion current which, in
turn, becomes an error in measured particle concentration. Other unipolar ion
sources appear to have similar or greater problems. For example, traditional
high voltage corona discharge devices are difficult to control such that they
produce a constant supply of ions. They also expose the particles to a high
electric field, causing many particles to collect on the walls of the device.
This results in rapid contamination of the instrument, and an error in the
measurement since some particles do not escape the ionizing chaither. The radio-
active ion source also utilizes an electric field, but the field is much lower
in intensity resulting in considerably less particle collection in the ionizing
chamber.
However, since the radioactive sources used in ambient air monitors appear
too weak to supply enough ions for stack particle concentrations (see discussion
below) alternate designs may be necessary. Sample dilution upstream of the
sensor is one possible solution. Another is to find other radioactive materials
which result in higher ion production. A third alternative is to use properly
designed corona discharge ion sources.
Figure 2 shows schematically a number of possible designs conceived by the
authors for an ion current attenuation instrument using corona current ion sources.
Designs A — E use primarily field charging while design F uses diffusion charging.
Thus, designs A — E would place charge proportional to 2 (surface area) on the
particles if the particles become charge saturated. Design F wo ld place charge
proportional to about D L 2 if the particl became charge saturated.
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— “GAS STREAM VELOCITY IN A-C
SHARP DISK
r%
L -
B
C
Figure 2. A, B, C.
Other ion current attenuation designs.
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— GAS STREAM VELOCITY IN D AND E
COMPRESSED AIR
CURRENT AMPLIFIER
AND INDICATOR
Figure 2. D, E, F. Other ion current attenuation designs.
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D
E
F
ROD
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Designs C and D would minimize particle deposition in the charging area.
The mobility of the ions is at least an order of magnitude greater than most
of the particles. Thus, the aerosol velocity can be chosen such that ions
travel from the corona wire to the current sensing rods while the charged
particles pass through. The other designs would definitely require periodic
cleaning. Designs C and D may need cleaning, but much less often.
Designs B and F subject nearly all the particles to identical charging
conditions while the other designs do not. However, design B will probably
have large particle deposition, causing a measurement error and instrument
contamination. Design F depends on the particles remaining airborne for the
time required to pass completely through the charging chamber. This may be
on the order of 10 — 60 seconds, long enough for large particles to settle
out due to gravity. Some of the particles will also be lost to the walls because
of the mutual repulsion of the charged particles.
Design F closely resembles the particle charger on the Whitby aerosol
analyzer (electrical particle counter) 68 . Design A resembles the ion drift
anemometer used for measuring the mass flow of a gas.
Designs A — E offer the possibility of easily detecting the ion current
attenuation caused by the particles. Figure 3 shows the basic scheme. The ion
current attenuation, which is directly proportional to particle concentration, is
AE ROSOL
Figure 3. Complete ion current attenuation instriment system
for designs shown in Figure 2: A—E.
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simply (I — 1 i • Design F cannot use the same scheme since much of the total
ion curre t is lost in the ionizer orifice area.
Any of these techniques may work, More detailed design and development is
needed to fully evaluate each concept. For use as a particle concentration
monitor where the mass of particles is the important parameter, ion current
attenuation appears to be one of the best candidatóa.
Application to Mass Emissions Measurement in Stacks
Figure 4 shows the current ratio of I/I compared with gravimetric con-
centrations for a of tests using quar z duet with a mean particle size
of 0.7 urn in Coenen’s t ’.O instrument. Notice the nearly Linear relationship and
good correlation above 1 mg/rn’. In these tests, very small condensation nuclei
were eliminated. Apparently, small weights of such nuclei can cause large
attenuation of the ion current, resulting in very poor correlation with gravi—
metric measurements, Since combustion particle sources, such as coal— and oil—
fired combustion facilities, probably emit large and varying numbers of such
nuclei, somewhat poorer correlation could be expected than that shown in Figure 4.
The high particle concentrations encountered in coal combustion emissions
would pose a problem for ion current attenuation instruments which are normally
used for measuring cleaner atmospheres such as outdoor city pollution. Figure 4
indicates about 30% attenuation with less than 10 mg/a 3 concentration of quartz
dust. At concentrations normally encountered in coal combustion emissions
(100 mg/in 3 ), extrapolation of Figure 4 indicates almost complete attenuation of
the ion current. Presumably, good engineering design could improve such an in-
strument for stack concentrations, but some dilution would probably be necessary.
The other proposed designs would presumably operate at higher concentration levels,
but further experimental and theoretical work is needed to define the operating
range.
• Mcasuremcntin front
,
K
U
N
C
or thc (titer
• Measurement behtnd
the (t iter
-
\
i
i
I
I
I !. 1_
9 1 2 3 4 iJ
Gravtmetrtc concentration C
Figure 4. ‘Gravirnetric calibration of ion current atteitu. 5 J 8 n
instrument on quartz dust according to CoenenL
IL,
.1
10
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The difficulties in measuring relatively small current levels with an
electrometer may be a problem for stack use. Current levels of i — to 10—10
amperes could be expected. The insulation of the central electrode in stack
environments could probably be done but would require careful design.
Ion current attenuation will probably never yield an output response which
correlates well with particle mass concentration. Thus, it cannot be recommended
for such measurements. However, the technique appears to offer several important
features for other particle concentration measurements, including instantaneous
response proportional roughly to particle surface area concentration, another
important particle parameter which is more sensitive to submicron particles than
is particle mass. The technique offers design simplicity with corresponding low
cost. The technique requires point sampling, a necessary feature for many
particulate emissions measurements. These features recommend this technique
for further research and development to overcome the operational difficulties
discussed above.
Application to Particle Size Measurement in Stacks
The ion current attenuation technique does not appear capable of measuring
size distribution of particles by itself. However, it could be used as a particle
concentration detector downstream from a particle size classifier, such as an
impactor or electrical mobility separator.
The ion current attenuation monitor is very sensitive to particles below
0.1 urn and less sensitive to larger particles. Thus, there may be a problem using
this technique behind an impactor since impactors operate in the range above 0.3 urn
at ambient pressure. In stacks with relatively small numbers of particles below
0.1 urn, such a combination holds some promise. However, if considerable numbers
of particles exist below 0.1 urn, particularly if the concentration of such particles
fluctuates, they may completely mask out the changes in ion current due to the
impactor. Other, more promising, methods exist for measuring particle size distri-
butions in this range.
If an electrical mobility separator (described In the next section) is used
to classify particles by size, a simpler, more accurate method exists for
measuring particle concentration. The next section on Ion capture discusses
the better method.
ION CAPTURE
Another way of using the charging of particles passing through an ion cloud
for measuring aerosol particle concentration is to measure the ion current carried
away by the particles. This can be done, as shown in Figure 5, by collecting the
particles, either by filtration or by electrostatic precipitation, and measuring
the current draining from the particles to ground with an electrometer. Several
such instruments have been developed.
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AEROSOL
STREAM
I ., )
Figure 5. Basic requirements for ion capture measurements.
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554,122,552 -
Grindell reported on the electrostatic dust monitor shown in
Figure 6. The sensing portion of the monitor is located outside the stack.
As flue gas passes through the monitor, particles first become charged by a
corona discharge. The particles then pass into the collecting section where an
electric field precipitates them onto the tube wall. The charge carried by the
particles then passes through an electrometer to ground. The measured current,
a direct indication of particle surface area passing through the monitor per
unit time, can be amplified and recorded.
The monitor responds automatically to changes in stack gas velocity because
of the unique suction system. Flue gas passing the exit cone causes a suction
inside the cone. Since the electrostatic system causes very little resistance
to gas flow, little suction is needed. By correct design of the exit cone and
the inlet nozzle, nearly isokinetic sampling can be automatically obtained over
a reasonable range of stack gas flow conditions, a very desirable feature.
This suction system has the additional feature of making the measurement
proportional to particulate emissions per unit time rather than just particle con-
centration such as most monitors measure. As the velocity of gas past the exit
cone increases, the rate of suction automatically increases. Thus, the instrument
Fig. 6. An ion
544
capture design from Crindell.
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responds to the product of particle concentration and volumetric gas flow rate,
yielding a direct measurement of the particulate emissions rate . This would
simplify interpretation of data from stacks with changing flow conditions.
Since the charging mechanism here is primarily field charging, the monitor
is sensitive to particle surface area). 2 U Grindel]? 22 ’ 554 points out several
advantages to surface area measurements, and discusses, in some detail, the
theoretical basis for the monitor.
The Grindell monitor cleans itself periodically by rapping the collector
electrode and purging the monitor with a blast of relatively clean, outside air.
The purge air enters the monitor through a wide mesh which also ormá the ground
electrode from the corona discharge. Grindell reports little particle loss in
the charging region during the measuring operation. However, what does collect
in the charging region as well as what deposits elsewhere, including the precip-
itating region, is cleaned Out well by rapping and purging for about 30 sec every
20 mm.
The monitor must be heated above the dew point of the vapors in the stacks.
An earlier Grindell monitor was designed for mounting inside the the stack,
eliminating the need for external heating. However, because of installation
difficulties, he mounted models outside the stack and supplied 700 watts to the
outside casing of the body and the exterior pipes to raise the temperature to
about 100°C. Reasons for heating the monitor are:
a) To prevent excessive corrosion of metal parts,
b) Damp fly ash contaminates the monitor and will not come off
with an air blast,
c) Moisture interferes with the electrical insulators, and
d) Electrolytes in contact with dissimilar metals cause a
battery effect giving unwanted signals to the amplifier.
The monitor has been tested in a stack with coal combustion emissions. It can
operate for weeks without human attention. The largest operational problem appears
to be long term deterioration and contamination of the apparatus.
Slone, et al, 302 .describes the construction and testing of a similar instrument
on a coal—fired boiler stack. Although the prototype instrument had several minor
design weaknesses, they report positive results. The report stresses that the
instrument measures particulate surface area and does not make a strong attempt to
correlate the measurement with particulate mass.
Any of the alternate design concepts shown in Figure 2 may be modified so that
the charge placed on the particles is monitored. The same problem and features
discussed for ion current attenuation would be present in an ion capture instrument.
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Whitby and McFarland 8 suggest a modification of this technique to measure
the mass concentration of particles in ambient air. If the particles are charged
by diffusion and the high mobility, 8111811 particles are separated out of the main
aerosol stream, the resulting ion current carried by the remaining particles will
theoretically correlate better with mass concentration. The principle requires
the shape of the particle size distribution to remain constant, which is the
reason for removing the small particles which have rapidly fluctuating concentration.
Figure 7 shows a schematic of the monitor with a mobility separator. Whitby and
McFarland found a significant improvement in the correlation of the measured
parameter with volume concentration for atmospheric aerosols sampled with an instru-
ment using diffusion charging. The charge per particle placed on the particles by
this instrument is proportional to D 1.2 which makes the measurement quite
sensitive to large numbers of small particles, even with the smallest of them
removed.
The fluctuations in particle size distribution within stacks is not experi-
mentally known. However, relatively large fluctuations can be expected on a short—
term (10 sec) basis, even in the particle size range above 0.1 um. Thus, little
improvement in the correlation with mass could be expected with the addition of a
mobility separator to the monitor.
Whitby and Clark 68 have successfully used the mobility separation technique
described in Figure 7 to measure the size distribution of submicron aerosol
particles. In their system shown in Figure 8, the mobility separator can be
adjusted manually or automatically to separate out particles with mobilities
above preset values. The mobility of a particle is, in turn, inversely
proportional to particle diameter for particles below 1 im. The commercial
version of the electrical particle counter can measure number concentration as
a function of particle diameter for 14 size ranges from 0.006 to 1 urn. No
other instrument is known which can automatically measure size distributions in
this size range.
,tirn a, r &IC i Jt’
-------
AEROSOL 1’ 1 I
:: ::: ELECTROSTATIC _______ MOB LITY _______ PARTICLE I SUCTION
CHARGER / SEPARATOR COLLECTOR 1 / I SOURCE
StREAM ____________ ____________ _____ J ____________
m ______ ______ _____________
ELECTROMETEH _____ RECORDER
I______ ___________
“3
Figure 7. Modified ion capture design using a mobility
separator from Whitby and McFarland 8 .
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Figure 8. Modified ion capture design used to measure
submicron particle size dietribution.
Application to Mass Emissions Measurement in Stacks
The correlation of the measurement made with Grindell’ a monitor with mass
emissions is not expected to be very good. The monitor is sensitive roughly to
particle surface area and directly to the gas flow rate if it has the automatic
suction system.
The monitor appears to have a number of positive features, however. It gives
a continuous—measurement of emissions rate rather than quasi—continuous measure-
ment of mass concentration as most instruments do. It appears possible to design
a monitor to operate inside a stack, eliminating the need for dilution or heating
systems. Even with sample extraction, the heating system is very simple. The
monitor can be made rugged and can clean itself easily. The monitor should be
relatively inexpensive, perhaps in the $5,000 range. At least 2 monitors have
been built and operated within coal—fired boiler stacks with no apparent major
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problems. They were operated for weeks without human attention. O tpüt data is
analog and continuous, making recording, or other data manipulations, simple. The
monitor measures particle surface area which is more sensitive to small particle
number concentration measurement. Thus, particle area is a compromise parameter
which responds somewhat to all particle sizes rather than just the large or the
small ones.
This technique will not measure particle mass very well. However, if particle
surface area is of interest, this technique is one of the most promising for
continuous monitoring.
CONTACT CHARGE TRANSFER
When particles hit or slide along a surface, they usually transfer electrical
charge to the surface. This principle has been used in the design of several dust
concentration measuring instruments. Three such instruments are discussed below:
a. the Konitest ,
b. the Probe—in—Nozzle Technique , and
c. the Particle Bounce Technique.
Konitest
The Konitest uses the electrical charge transfer caused by particles sliding
along a wall as a measure of aerosol particle concentrations, Figure 9 shows the
complete measuring system as it samples from a duct. Figure 10 schematically shows
the details of the Konitest sensor. As the aerosol stream is sucked into the
sensing exciter tube, the stream is given a whirling motion by the tangential in-
let duct. Centrifugal force throws particles against the exciter tube wall where
they slide through the seüsor in a helical path. Electrical charge generated at the
exciter tube wall leaks to ground through a measuring amplifier. The amplified
signal can then be recorded, e.g., by a strip—chart recorder.
Electrical heating
Supporting Insulation/ Exciter tube
tube ____________
measuring UU
ft - chamber )measuringchamber
measurement Pen recorder
Fig. 9. Complete Konitest stack monitoring system from
Schutz 659 and Prochazka. 923
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r
x ELECTRICALLY
FLOATING TUBE
RECORDER
D.C. AMPLIFIER
1
Fig. 10. Detail of Konitest sensing head showing particle
trajectories from Prochazka 923
A. later version of the Konitest, shown in Figure 11, uses a venturi nozzle
as the sensing exciter tube. With equal dust concentrations, the nozzle yields
lower current levels than the centrifugal device. However, the current levels
are still high enough to amplify and record with most dusts.
Both versions use the Konitest sensor as a gas flow measuring instrument
simultaneous with the dust concentration measurement. This is done with the
differential pressure measuring systems shown in Figure 9 and 11. The exciter tube
material is steatite, a semiconductor substance, also known as magnesium hydro—
silicate. The exciter tube must be electrically insulated from the outer housing
so that leakage currents can be detected by the amplifier.
Fig. 11. Alternate Konitest design using an electrically
floating venturi as the exciter tube rather than
the cyclone from Prochazka. 923
v1
SECTION XX
Exciter tube
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Both the inlet tube and sensor itself are electrically heated to prevent vapor
condensation. The dust passing through the instrument must be dry or it will
stick to the exciter tube. Schutz 659 reports that very thin coatings of particles,
just a few microns thick, cause severe changes in measured values, at times even
reversing the current polarity. Duwe1 225 and Konig and Rock 24 ’ report that the
Konitest calibration changes with temperature. Thus, not only is it important to
heat the device to reduce contamination but the temperature must also be kept
constant, With thermostatically-controlled electric heaters, temperature control
should not be a major problem. However, Schutz suggests that long term operation in
dusts containing many small particles is difficult because of contamination of the
exciter tube.
The electrical charge generated by the particle—surface contact depends on both
the surface material and the particle composition. Some materials yield positive
polarity currents while others yield currents of negative polarity. Certain combi-
nations of dusts may yield no current. Thus, each different type of particle requires
individual calibration. The composition of coal combustion emissions is probably
constant enough so that this problem would not become overwhelming, but this
certainly must be checked experimentally.
The particle size distribution and the air flow rate through the instrument may
affect the contact of particles with the exciter tube, and, thus, affect the cali-
bration of the instrument.
In spite of the many factors affecting the measured value of the Konitest,
several investigators 225 , 241 , 659 , 923 report excellent correlation with particle
mass concentration if the instrument is calibrated for each new condition. Figure
12, from Prochazka 923 , shows the indicated current versus gravimetric particle con-
centration for flue gas from a power station with concentrations of 0.1 to 2.2
gm/in 3 . The data was Obtained by determining an average Konitest current during the
time necessary to collect enough material on a filter so that it can be weighed.
The filter was immediately downstream from the Konitest, eliminating any error due
to spatial variation in dust concentration in the duct. The data shows very little
scatter,
1•ir’A
AJ
T
:fi
:J
I
i::: : :
; . J
H
-
——1-—
-—--f—4
,
1 7
i: .::
P
- —-
—
-
‘1 .2:13 Nov.64
‘j:24Febr6 ”
•9 ?25Febr 5———-
‘t:26Feb 6 —_-.
.7 :27 Feb’r .65 — — -
‘ U:3O June 6
11111
160
1
7:
50
50
29
- DJoicJacuuiz7au.gwtg u t .vt uu&iuu ,z ,j
Dust content
Fig. 12. Gravimetric calibration of Konitest for flue
gas from Prochazka. 923
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1189
A very recent program by Schnitzler, et al , extensively evaluated the
Konitest, two light transmission instruments, and two beta attenuation instruments
In a modern plant fired by coal. Figure 13 shows the comparative results of
the five instruments in ternis of manually measured gravimetric concentration as a
function of instrument reading. Table 1 indicates the results of these tests.
The dark lines In Figure 13 are the calibration curves obtained by drawing lines
through the data points. The lighter parallel lines on either side of the
calibration line denote extrapolations of the 95% confidence level calculated for
the 150 mg/ui 3 concentration level.
Each of the calibration curves shown in Figure 14 were obtained from one
instrument operating In one effluent duct. The data is the average of measure-
ments made with three significantly different plant operating conditions: 1) with
soot—blowing, 2) without soot—blowing, and 3) with 45% of plant operating capacity.
A look at the calibration curves for each of these conditions can show how well
the measured parameter correlates with mass, I.e., how much error can be expected
if the effluent conditions change from what they were during calibrations.
The beta attenuation instruments, which sense a parameter closely related to
particle mass, show excellent correlation with gravimetric mass measurements • How-
ever, notice the surprisingly good mass correlation of the KonItest and the second
light transmission instrument which was manufactured by Sick in Germany.
TABLE 1. Stm mary of experimental calibration of five instruments
1189
by Schnitzler et al
Instrument
Tranamissometer
Durag Sick
A B
Contact—
Electrostatic
Konitest
‘
Beta Radiation
Attenuation
A B
Number of Measurements
312
292
312
29 43
Correlation Coefficient
0.63
0.87
0.92
0.94 0.8].
Measuremen Tolerance at
150 mg/Nm
±68%
+39%
±22% ±31%
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TRANSMISSOMETER. A
I)
mgNm /
I
1
J”
300
250
—
—
—
— — —
‘
—
—
71
7
200
is o
:-
74
-
74
b c
s6
A
-
74__ __
-
-
— —
—
—
.
I ,
U
14
U
w
.14
(ng
350
g v
30 17
—
—
—
—
—
—
i50
7/f
- -
--
b c
(
-
--
•50
•
/
U
— — —
0
10 iS 20 2
30. - 0 5
10 15 20
75 30
Instrument Reading Instrument Reading
(Arbitrary Units) . . (Arbitrary Units)
U
.,-4
14
1.4
S
C ’,
14
C .,
BETA A
—
Figure 13. Gravimetric calibration of two light transmi someters,
one Konitest, an 1 two beta radiation attenuation
instruments in a coal—fired effluent duct from
Schnitzler et a ! ” 89 .
TRANSMISSOMETER B
‘Mm 3
I
C)
14
U
4)
.
14
C.)
KONITEST.
Instrument Reading
(Arbitrary Units)
U
14
1 . 4
4)
C’,
14
C.)
BETA B
C)
I -I
U
14
(B
Instrument Reading
(Arbitrary Units)
Instrument Reading
(Arbitrary Units)
THERMO-SYSTEMS INC..
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—13].—
Figure 14 is a comparison of the calibration curves for each of the
three operating conditions for the Konitest and the two light transmission
instruments. Notice that the three Koniteat calibration curves fall nearly
on top of each other, indicating little effect on the mass correlation due to
the changes in plant operating conditions. The two light transmission instru-
ments show considerably different curves for each of the three operating
conditions.
T.IANSMISSOMETER A
TRANSMISSOMETER B
350
300
250
200
t50
/
d
7-
--
—
--
-
100
s6
.
—
——
3
,
0
-
--
--
-
--
—
—
0
I
——— —
4
j
;
I
(
0 5 10 15 20 25 30
KONITEST
mg/Nm’
400
350
.300
- - - I
- -I
-
; . I,
200
7
7.
150
100
so
C
o $ 10 5 20 25 30
Ins tr mLent Reading
(Arbitrary Units)
Instri.mient Reading
(Arbitrary Units)
Figure 14.
Gravimetric calibration of two light transmissometers
and one Konitest in a coal—fired effluent duct under
plant operating conditions of:].) with soot blowing,
2) wjthout soot blowing, 3) with minimum load
from Schnitzleret al
TH ERMO- SYSTEMS INC.
mg ‘Nm 3
Instrument Reading
(Arbitrary Units)
10 15 20 25 30 35 40
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Apjlication To Mass Emission Measurement In Stacks
From data such as Figures 12 — 14, one is lead to believe that the Itonitest
responds quite well to particle mass. The literature give no hint of a theoreti-
cal reason for this. A possible reason is that the force driving particles toward
the steatite tube is centrifugal force, which depends on particle mass. This
force also regulates the number of collisions and the intensity of the collisons
of particles with the wall.
The concentration range of the ins truxaent is reportedly from less than 100
pgm/m 3 to greater than 1 gm/a 3 . This certainly covers the entire range normally
experienced downstream of a collector in a coal— or oil—fired combustion facility.
The response of the Konitest is essentially instantaneous. The electrical
signal can be monitored with its natural, fast response or it can be damped
electrically yielding a more steady concentration value.
The major problems with the Konitest for measuring particulate emissions
from coal— or oil—fired combustion facilities are: 659
a) Long—term contamination of the exciter tube, primarily by
submicroâ particles,
b) Rapid cbanges in the instrument indication, caused by large
fluctuations in submicron particle concentration, which do
not represent real changes in particle mass loading, and
c) The measured parameter is not true particle mass, although
on—site calibrations can make resonable correlation possible.
d) Changes in particle composition can change the calibration
drastically, even causing the readout current to reverse
polarity.
Although we do not fully understand why they occur, the good mass correlation
reported from several test programs are impressive. The investigators report no
major operational problems with the Konitest. The device requires calibration in
each different installation, but its fast, nearly instantaneous, rçsponse and
its good calibrated mass correlation recommend it for some monitoring applications.
More information should be obtained from people with operating experience and, if
their recommendations warrant it, considerable work appears jusitifed to develop
both the theory and the hardware for this technique. The problems listed above
could limit the use of the Konitest in coal—fired stacks quite sev ’rely.
Until recently, the Konitest was conunercially manufactured by 3. C. Eckardt,
Stuttgart—Bad Cannstatt, West Germany. However, the company no longer manufactures
them.
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Probe—In—Nozzle Technique
Schutz 659 ’ 684 ’ 922 ’ 1036 describes another contact charge transfer instrument
which consists of a conical metal probe in the throat of a venturi as shown in
Figure 15. Particles passing through the nozzle hit the probe because of their
high inertia caused by flow velocities above 100 rn/sec. When the particles hit
the probe, a charge transfer occurs, similar to the charge transfer in the Konitest
instrument. The current draining from the probe is an indication of dust con-
centration. The current signal can be amplified, linearly or logarithmically,
and recorded by a strip chart recorder. As with theKonitest, the particle
parameter which is being sensed, i.e., area, mass, etc., is not clearly defined
but experimentally appears to be related to particle surface area.
Cheng, et al, 2 ° and Soo, et al, 5 ’ 104 describe measurements made with a
device apparently using similar principles,
Blower
659
Fig. .15. Probe—in—nozzle design from Schutz.
Application To Mass Emissions Measurement in Stacks
Several problems make the instrument questionable for mass emissions measure-
ment from coal— and oil—fired combustion sources.
A reasonable portion of the particles striking the probe must be large, i.e.,
greater than 10 jim, in order to keep the probe clean. If all particles are less
than 10 m with a significant portion below 1 tim, the probe will become dirty and
change the calibration of the instrument. Also, because all submicron particles
do not strike the probe, the instrument may not detect such particles reproducibly.
The curren levels measured from the prototype instruments was In the range
of 10—8 to l0 amperes. Such low current levels require careful instrument
design. The particles must be dry to prevent current leaks and also to prevent
excessive contamination of the probe. Schutz reports the maximum temperature at
which the instrument can operate is 70°C, considerably below the 175°C normally
enàountered in power plant stacks. To cool the gas stream to 70°C without vapor
condensation requires dilution. The maximum temperature and dilution requirement
eliminates the possibility of mounting the sensing head directly in a stack, one
of the primary features of this technique.
i Icasuring head Flow meter Filter
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No instruments of this type have been commercially marketed.
Because of the rather major problems with this technique and because the
technique has no real advantage over other electrostatic techniques, we recommend
that further development emphasize the other electrostatic techniques rather than
this one.
Particle Bounce Technique
Coenen 226 describes a unique particle concentration measurement technique
whereby particles bounce between plates of a high voltage, electric condenser,
carrying electric charges with them. Figure 16 illustrates the principle. When
a particle enters the aIr gap between the plates, the e1Cctri field accelerates It
toward one side or the other, depending on its initial charge. When it strikes a
plate, it exchanges its charge and is repelled toward the other plate. This process
continues until the particle is carried out from the condenser by the a r flow. With
each bounce, the particle carries charge from one condenser plate to the other, caus-
ing a net current to flow between the plates, The current level, measured by an
Condenser probe
iTT -
.
;
Ampi. E-iV.
Fig. 16. Illustration of particle bounce technique from
Coenen. 226
electrometer, is an indication of the concentration of
current can be amplified, and then recorded by a strip
Figure 17. The charge per particle is related roughly
The technique works best with large particles (>lQO m)
for some particles as small as 1 nn.
the particles. The indicated
charge recorder, as shown in
to particle surface. area.
but reportedly works well
Fig. 17. Particle bounce instri ent design from Coenen 2 !
1 Recorder Electrometer Dynamic condenser
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Application To Mass Emissions Measurement in Stacks
This technique has most of the same problems as the Konitest discussed
earlier. In addition, the particles should be primarily above lO im, above
the range of sizes in coal— or oil—emissions stacks.
No coimnercial units exist. No further development for stack monitoring is
recommended at this time.
CONCLUSIONS
Several electrostatic techniques show considerable promise for measurement
of effluent particle flow rates. One correlates well with mass concentration and
two measure particle surface area. These techniques are:
1. Konitest,
2. Ion capture, and
3. Ion current attenuation.
The Konitest, developed and tested in Germany, shows surprisingly good experimental
correlation with manually—obtained gravimetric measurements in several different
studies of effluent from coal—fired power plants. Although theory is not developed
for the device, the centrifugal force on the particles in the cyclone may account
for the good mass correlation. On the other hand, several other people report
poorly—documented operational problems with the instrument. Further work on the
development of theory and hardware appears justified because of the potential
advantages of the instrument.
A number of new designs are suggested for both the ion current attenuation
and the ion capture instruments. These instruments would probably respond to
particle surface area (D 2 ) with field charging or with diffusion charging.
An interesting suction t chnique, useful only on lo pressure drop devices such
as these, makes the instrument respond to particle flow rate rather than particle
concentration. The instantaneous readout of a wide variety of particle con-
centration levels is a positive feature of these instruments. Further development
work appears justified on these electrostatic techniques.
REFERENCES
20 Cheng, L., Tung, S. K., ana Soo, L. S., “Electrical Measurement of
Flow Rate of Pulverized Coal Suspension”, ASME Paper No. 69—WA/Pwr—l ,
14. p. (1969).
226 Coenen, W., “A New Principle of Recorded Dust Measurement and Its
Technical Application in a Battery—Powered Instrument”, Staub—Reinhalt
der Luft (Engi. Trans.) , V. 27, no. 12, p. 32—40 (Oct 1967).
THERMO-SYSTEMS INC.
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—a. 6—
245 Coenen, W.,..”Registrierende Staubmessung nach der Methode der
Kleinionenattlagerung”, Staub—Reinhalt der Luft , V. 24, no. 9,
p. 350—353 (Sep 1964).
284 Coenen, W.., “Staubmonitor zur betrieblichen Staububerwachung”, Staub—
Reinhalt der tuft , V. 23, no. 2, p. 119—123 (Feb 1963).
1040 Coenen, W., “Dust Measurement and Recording by the Method of Small Ion
Accumulatioti”, Staub—Reinhalt der Luft , V. 24, no. 9, p. 350—353
(Sep 1964).
225 Duwel, L., “Latest State of Development of Control Instruments for
the Continuous Monitoring of Dust Emissions”, Staub—Rejnhalt der Luft
( Engl. Trans..) , V. 28, no. 3, p. 42—53 (Mar1968).
122 Grindell, D. H., “An Electrostatic Dust Monitor”, lEE Proc. , Series A
(34), V. 107, p. 353 (1960).
552 Grindell, D. H., “Atmospheric Pollution by Solid Particles, Measuring
Significant Particle Surface Area by Charge Transfer”, Engineering ,
V. 187, p. 1350—1351 (Mar 1959).
554 Grindell, D. H., “Monitoring Smoke and Flue—Dust Emission”,
V. 2, no. 5 (1962).
242 Haeenclever,D., and Siegmann, H., “Neue Methode der Staubinessung
mittels Kleinioneanlagerung”, Staub—Reinhalt der Luft , V. 20, no. 7,
p. 212—218 (Jul 1960).
241 Konig, W., and Rock, H., “Untersuchungen am elektrostatlschen
Staubgehaltsmessgerate Konitest”, Staub—Rejnhalt der Luft , V. 21,
no. 8, p. 355—356 (Aug 1961).
581 Mohnen, V. A., and Holtz, P., “The SUNYA—ASRC Aerosol Detector”,
APCA Journal , V. 18, p. 667—668 (Oct 1968).
923 Prochazka, R., “Recording Dust Measurement With The Konitest”, Staub—
Reinhalt der Luft (Engi. Trans.) , V. 26, no. 5, p. 22—28 (May 1966).
659 Schutz, A., “Possibilities for Recorded Dust Measurement and Dust Control”,
Staub—Rejnhalt der Luft (Engl. Trans.) , V. 26, no. 10, p. 1—8 (Oct 1966).
684 Schutz, A., “Eine Anordung zur Registrierenden Kontaktelektrjschen
Staubmessung”, Staub—Reinha].t der Luft , V. 24, p. 359—363 (1964),
922 Schutz, A., “A Recording Dust—Measuring Instrument Based on Electric
Contact, with Logarithmic Indication”, Staub—Rejnhalt der Luft (Engi.
Trans.) , V. 26, no. 5, p. 18—21 (May 1966).
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1036 Schutz, A., “An Arrangement for Continuous Recording Dust Measurements
by the Contact—Electrification Method”, Staub—Reinhalt der Luft , V. 24,
no. 9, p. 359—363 (Sep 1964).
1189 Schnitzler, H., “Messtand f tr die Prufung und Kalibrierung von
Registrierenden Staub — und Gasmeasgeraten in eine Steinkohlengefeuerten
Kraftwerk”, SchrReihe Ver. Wags. — Boden Lufthyg. Berlin—Dahlem , V. 33,
Stuttgart (1970).
302 Slone, T. J., Lagarias, J. S., and Schiaffer, G. C., “Stack Effluent
Monitoring for Power Stations”, Reprint No. 285, American Inst. Co., Inc.,
Silver Spring, Md.
5 Soo, S. L., Stukel, J. J., and Hughes, J. M., “Measurement of Mass Flow
and Density of Aerosols in Transport”, Environmental Science and
Technology , V. 3, no. 4, p. 386—392 (Apr 1969).
104 Soo, S. L., Trezek, G. J., Dlmick, R. C., and Hohustreiter, G. F.,
“Concentration and Mass Flow Distributions in a Gas—Solid Suspension”,
Industrial & Engineering Chemistry Fundamentals , V. 3, p. 98—106 (1964).
8 Whitby, K. T., and McFarland, A. R., “Electrical Measurement of the
Mass Concentration of a Self—Preserving Aerosol Size Distribution”,
APCA Journal , V. 18, no. 11, p. 760—764 (Nov 1968).
68 Whitby, K. T., and Clark, W. E., “Electric Aerosol Particle Counting and
Size Distribution Measuring System for the 0.015 to i i Size Range”,
Tellus , V. 18, p. 573—586 (1966).
1211 Whitby, K. T., and Liu, B.Y.H., Chapter 3, Aerosol Science , Davies, C. N.,
editor, Academic Press, New York, (1966).
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LIGHT TRANSMISSION
by: B.Y.R. Liu and K. T. Whitby
INTRODUCTION
When light, or other electromagnetic radiation, passes through an aerosol,
its intensity reduces because of scattering and absorption by the particles.
The transmission or attenuation of light through an aerosol, therefore, can be
used as a measure of aerosol concentration.
Figure 1, according to Hodk1nson 211 shows the basic requirements of an
instrument for measuring light transmission through aerosols. Pin—hole apertures
are needed in order to prevent scattered light from reaching the photocell.
Pin-hole Dust porticles Pan—hole
Condenser Collimotor Telescope Photo-
lens lens lens cell
Fig. 1. Basic requirements of instrument for light 1211
extinction measurement according to Hodkinson.
Since the scattering and absorption of light by a particle is dependent upon
the geometrical and optical properties of the particles, light transmission measure-
ment alone generally does not provide sufficient information to determine the aerosol
mass concentration. In order to measure mass concentration by the transmission of
light through an aerosol, the density, the refractive index, the shape and the size
distribution of the particles must remain constant. For such aerosols the relation-
ship between mass concentration and light transmission usually must be established
by calibration.
Despite this rather severe limitation in terms of mass concentration measure-
ment, light transmiásion has remained one of the most widely used principles for the
continuous monitoring of smoke emission from stacks. The method is relatively
simple, and the apparatus involved can be comparatively inexpensive. Further, the
light beam is usually projected across a portion of the stack so that a reading is
obtained of the average particle concentration in that portion of the stack, and
the measured parameter relates closely to the visual appearance of the stack
effluent, one of the principal concerns in the monitoring and control of emission
from stacks.
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When combined with a suitable particle size classification device, light
transmission measurement can also be used to determine the particle size distri-
bution. One such device, the photosedimentometer, combines light transmission
measurement wtih a liquid sedimentation cell to determine the size distribution
of powder particles dispersed in a liquid. Similar principles can be considered
for possible application to size distribution measurements in stacks.
Although light transmission measurement is relatively simple to make, its
results are often difficult to interpret because the particle size is often of
the same order of magnitude as the wavelength of light used, and the scattering
of light by particles whose size is comparable to the wavelength of light is a
rather complex phenomenon. Therefore, in the following sections we shall briefly
review the fundamental light scattering properties of small particles from the
standpoint of stack monitoring by light transmission measurement.
BASIC PRINCIPLES
Consider a parallel light beam passing through a fluid medium containing
suspended particles. The transmittance, T, of the medium, i.e., the ratio of
transmitted light intensitX to incident light intensity, is given by the Bouguer,
or the Beer—Lambert, law, 1 ’ 11
T — exp (—ki) (1)
where k Is the extinction coefficient, or turbidity, and £ is the thickness of
the medium. Sometimes, the measured transmittance is expressed in terms of
optical density defined as
O.D. Log (LIT) (2)
instead of the transmittance or the turbidity. Consequently, instruments and
methods for aerosol measurement based upon light transmission principles have
been referred to variously under such names as transmissometer, smoke density
meter, photo—extinction measurement, turbidimetric measurement, etc.
The extinction (scattering and absorption) of light by a particle is usually
described in terms of a particle extinction coefficient, K, defined as the ratio
of the flux scattered and absorbed by the particle to the flux geometrically
incident upon the particle. If A is the projected area of a particle against the
light beam, then the product
A KA (3)
a p
defines a cross—section, Ae for total light extinction by the particle. The
transmittance of a medium of thickness £ is thus given by
T exp (—L E Ae) exp (—& Z K A ) (4)
THFRMO.SVSTFMS INC
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i4O_
where the sum, E, is to be performed over all particles within a unit volume.
The turbidity, k, of the medium,
k=EA EKA (5)
e p
is thus seen to provide a measure of the total extinction cross—section of the
particles in a unit volume.
The particle extinction coefficient, K, is a complicated function of the
size, shape and refractive index of the particle. It is also dependent upon
the spectral energy distribution of the light beam and the spectral sensitivity
of the photodeteàtor if it is to be related to the transmittance measured by the
light source—photodetector combination. However, in the geometrical scattering
region where the particle size is large compared to the wavelengths of radiation,
the extinction coefficient, K, assumes the constant value of 2. Under such
conditions, equation (5) shows that the turbidity is a measure of the total
projected area of the particles in a unit volume. For particles which are reasonably
convex in shape and orientated at random, their projected area isap roximately
equal to 1/4 their total surface area according to Cauchy’s Theorem. 211 Thus
k =(l/2)E A ‘(l/2Xp E V) (6)
where A is the external surface area of a partiàle, V is the particle volume,
(p E V) is the mass concentration of the aerosol and
D EV/ZA (7)
is the volume—surface, or the mean Sauter, diameter of the particles. Thus, we
see from equation (6) that, in the geometrical scattering regime, the turbidity
of the medium is directly proportional to the mass concentration of the aerosol
only when the volume—surface mean diameter of the aerosol remains constant. On
the other hand, for the same mass concentration, the turbidity, or the optical
density of an aerosol, is inversely proportional to. the mean volume—surface
diameter. Figure 2, showing the results of a study by Lucas and Snowsill 485 on
the response characteristics of a smoke density meter, seems to confirm this
conclusion.
Theoretically, the particle extinction coefficient, K, will approach the
constant value of 2 only in the limit when the particle size parameter a; rrD/X-*co,
where D is the particle diameter and A is the wavelength of light. From a practical
point of view, K will have reached the value of 2 to within about ± 20% when a
exceeds a certain critical value. Table 1 shows an estimate of this critical a
value and the corresponding minimum particle size for A 0.5 I.Lm for various types
of particles. These are rough estimates based upon our pre8ent knowledge of the light
scattering properites of particles. More accurate estimates on some of the values
can be obtained by further calculation.
THERMO-SYSTEMS INC.
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:
•1
•1•.
0
-I
U
Fig. 2. Response of smoke density meter to dust particles of
different sizes according to Lucas and Snowsili. 485
Aerosols
Monodisperse spheres
and D in order that
K, is within ± 20%
D (X 0.5iim)
mm
9 l,5um
20 3.3 im
1 0.17 irn
2.5 0.42 urn
5 0.84 urn
10 1.7 urn
is
1C3
so
so
2i
I
PARTICLE DIA! ET I,..
Table 1 .
Estimated minimum value for a —
the particle eztinction.coefficient,
of the constant value, 2.
a
mm
in 1.33 (water)
in — 2.0 (many mineral particles)
in — (totally reflecting particle)
in — 1.55 — 0.66 1 (highly absorbing
particles such as carbon black)
Irregularly shaped particles and
polydisperse aerosols
in — 1.5
in — 2.0
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It is interesting to note in Table 1 that for light transmission measurement
in stacks where the aerosol is undoubtedly quite polydisperse, tranamiasometers
or smoke density photometers will operate essentailly in the geometrical scattering
regime for particles larger than 1 to 2 inn diameter. For highly absorbing paiticles
such as carbon black the geometrical scattering regime can be reached for particles
as small as 0.5 jim. This is a particularly significant conc].usion since in the
geometrical scattering regime the particle extinction coefficient, K, is independent
of the wavelength of the radiation. Thus, the turbidity and the optical density
of the aerosol will not depend on the wavelength for particles larger than 0.5 to
2 ‘rn. Hence, no information on the size distribution of the particles can be obtained
by varying the wavelength of the radiation within the limited range of the optical
spectrum and in the near Infrared if the particles in the stack are predominantly
larger than these minimum sizes. This point is discussed more fully in the
section entitled “Multi—Wavelength Light Transmission”.
Below the geometrical scattering regime, the light scattering properties of
the particles become quite complicated. The following paragraphs review briefly
the scattering properties of small particles.
A. Dielectric and Absorbing Spheres
For spherical particles, the particle extinct±on coefficient, K, is a function
of two parameters: m,the refractive index of the particle relative to the suspend-
ing medium, and ci irD/X, a dimensionless particle size parameter, defined as the
ratio of the particle circumference, rrD, to the wavelength of the.incident radiation A.
The complete theory for the calculation of K from electromagnetic theory for this
case was first formulated by Mie,’ 47 and treated n several well known texts and
papers including Stratton, 125 ° Van de Rulst, 1201 Kerker, 12 ]- 4 Kratohvil, 843 Born and
Wolf, 1212 Newton, 1 ]. 99 and Kerker. 1215 The calculation, requiring the summation of
many terms in an infinite series, has been performed by many authors for various
values of m and a. Published values of K derived from Mie theory calculations are
available in the literature. Summaries of the available calculation results have
been given by Van de Hulst 1201 (p. 166, 273), Hodkinson 121 ’ (p. 303), andmore
recently by Kerkerl 215 (Table 3.2, p. 78).
Figure 3 shows the e ctinction coefficient, K, for scattering by non—absorbing
dielectric spheres with refractive indices of 1.4 1.44, 1.486, 1.5 and 2.0 accord-
ing to the calculations of pefl dOrf,846,847,848,1j8l All curves show the character—
is tic feature of a damped oscillation, with the value of K reaching a peak at the
first maximum and eventually tending to the constant value of 2 as ct-+ . Super-
imposed on this damped oscillation are minor, ripples, which maybe ignored for all
practical purposes. A table giving the smoothed values of K for 0.8 < m < 2.0 and
0.2 < a < 20 is given by Penndorf. 200
ThERMOSYSTEMS INC..
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-143—
• - -
— :._ _ _j
2 _s
+ :
I I I 1 1 !Fi
-E 2
__________ I I I I i I I I I 1 L I
i5 20 25 30
Totoi Mic scatteziz g cocaicicnt K lots— 1.44
U function of tho size parnznctot a.
n-fl- rrr
L =
: : :‘::l:
ii•i•1 I I ii I I I I 1 j I
Ill liii Ii I I
1,1 I
K or ii — 1.50
i ion oi i c iz param tct a.
Tot,.1 Mle ,c&tt.ring coefficient IC for s- 1.486
u function of the size parameter a.
Fig. 3. Scattering coefficient for dielectric particles according
to Penndorf. 846 , 847 , 848 , 1181
Figure 4 shows the value of K plotted against the phase shift parameter,
p— 2 cs(m — 1), as presented by Kerker 1215 (p. 105). It shove that the scattering
curves are similar in shape and the maxima and minima are nearly equally spaced on
this K vs p plot, with the maximum loc*ted at approximately p — (n — l/4)2IT
— (n + l/6)2ir, where n is an integer. ‘ The peak scattering
and the minima at
coefficient,Vhich occurs at the
K — 2 — 4 siflp + cosp )
p
THERMO SYSTEMS INC.
, 4 I I
—1
. 1.2.0
LCw oA
P.niiCovl
Eit,, . a suet ?e . 1sldetf
- .-.-——-——— —
• 0
x
..
• I. •‘ I
I i .: I I I
I ______
j_fi L E IJI
Total Mic scattcring coefficient K for n—1.40
u function of the use parameter a.
f
•
;
I
r
:
i
i
I
I
03 4 5 S $0 ‘3 *4 4 * 4* 30
$135 fl*AMCTCS a
Total M ecat&.ring eoc c nts K ; fw cthn of thc M i s pzrametcr
. for ili. r. trsctive Index ii —2.00.
- EE
K
EII1ITI ITTT’IIIIIII
1..L.V.LLAiL4
U
J_.L.L.L.L_U.
J_I_L _LJ
,I I I
4
2
0
±A E 1 H:H+d±1bi__
10
I I I I
II
t : :I:i: :
II
‘I
Ii
ill I III
IS 20 25
m—l,to6form2. Forthe
scattering coefficient is given
first maximum, extends from approximately 3.2 for
case of anomalous diffraction when m 1 the
by the Van de Hulet equationl 2 O 1 (p. 176)
p
(8)
-------
- 144—
0
0
p
Fig. 4. Scattering coefficient of dielectric spheres plotted against 1215
the phase shift parameter p — 2 a(m — 1) according to Kerker.
Pd. designates the plotting increment.
For absorbing spheres the oscillation in the extinction curve becomes less
pronounced. Curve C of Figure 5, according to Hodkinson 1211 (p. 289), shows
that for an absorbing particle with the complex refractive index m — 1.55 — 0.66 i,
which would be appropriate for carbon particles in water, the oscillation has
6
m”Z.IOS, P!w .2
4
3
2
0
20
30
ThERMO. SYSTEMS INC
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C
U
4.
0
0
C
0
‘I
C
4.
Fig. 5. Extinction coefficient for absorbing and
dielectric spherical particles aczording
to Hodkinaon’ 21 ’ (p. 289).
—145—
completely damped out. The effect of absorption in a particle can be more clearly
seen in Figure 6 given by Allen , 33 calculated according to the following equation
given by Van de Hulst 1 • 20 1(p, 179)
(COSØ)2
p
— exp(—p tan 8) (c088)2
sin (p —.
cos (p — 8) + 4 (CO$8)2 cos 28.
(9)
Fig. 6. Extinction coefficient for absorbing(broken
Line)and dielectric(full.]ine)spheres calculated
by Allen 33 ac o 1 ding to the equation of
Van de }lulst 1 ’° (p. 179).
Rere the complex refractive index of the particle material La rn m’ + m’ I and
the loss angle 8 is defined as
tan B m’/(m’ — 1).
(10 )
Particle size pzrcmele, . d / X
K 2 — exp(- p tan 8)
2 4
6 8 10 1214 16 18 20
uw2x tm-1
THERMO-SYSTEMS INC.
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— a.., U—
Equation (9) is valid when ha — iI<< 1. Figure 6 shows that the oscillation in
the extinction curve is quickly damped out for finite values of B.
Figure 7 shows that for the case of the totally reflecting sphere (in co)
whose extinction coefficient can be represented by the empirical equation 1 . 201
—2/3 —l
K 2+0.07ct +0.42a
(11)
over the range, 6
-------
—147—
2 3 4 5 1
Particle size, 0
White light
Fig. 8. Smoothing of the extinction curve (in 1.33) groduced
by white light froni a tungsten source at 2700 K and a
selenium photodetector. 33
C. Irregular Particles
Figure 9 and Figure 10 show the scattering curves for various irregularly
shaped particles dispersed in a liquid as measured experimentally by
Hodkinson. 1211 ’ 1213 As expected, the oscillation in the scattering curves are
nearly completely damped out due to the random orientation of the irregular
particles. The approach to the geometrical scattering regime (where K 2) is
much more rapid as compared to spherical particles of the same refractive index.
I C
2.0
Monochromatic light
THERMO• SYSTEMS INC.
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—148—
,
—(-
p.’
•0
v” ”V, -si, -
—,
—fr--———
I
.
-
I
01•1,11 . .I1è 0
— FIia,11 0 —
C ,lu.h. ...1HS
T ,Idv.I...1104 I
— VI.,,s.i i&Ik ..IOU K —
V
I
10
Ph, ,. dilf,i.u,c, ps , .ts ’ p • 2.1. - I)
(b) Voilivi mlwsls ii ws*
‘ I ••° • •
H’
— — — —
S
I( aTIvf #r .
PiaC1IvI lVqlCT,
1151 133
1145 1 345
1.11$ l•
1•l l3fl
Ie3 14U
Phi.. dflsi.ic. pSI ilsI p. 3s( - 1)
NOT REPRODUCIBLE
(s) Q,srtz i vsnivs iwsv a .h$iss, m — 10S3 i
Fig. 9. Scattering coefficient for
shaped particles according
I
j
.1
j
Psnieh.si,. •..ivø.., . • id A
(.) Quartz porticlis in wotsr
‘a
I
I
I
IS
t•0
I
0
0
Quini hilt. a • IS O
irregularly 1213
to Hodkinson.
ThERMO- SYSTEMS INC.
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—149—
a,
&
Fig. 10. Scattering coefficient for irregularly shaped
particles according to Hodkinson. 121 l
APPLICATION TO MASS CONCENTRATION MEASUREMENT IN STACKS
Light transmission instruments are widely used for the continuous monitoring
of smoke emission from stacks. Such instruments are generally referred to as
smoke density meters. Their readings are usually expressed in smoke density units,
as defined in equation (2), or simply in units of percent transmittance. Some-
times the reading; are also expressed in equivalent Ringelmann numbers, where
Ringelmann 5 would be equivalent to 0% transmittance, Ringelmann 4, 20% trans-
mittance, etc. A list of domestic manufacturers of smoke density meters and alarms
is given below:
Airflow Developments (Canada) Ltd.
Bailey Meter Co.
Cleveland Controls, Inc.
Combustion Equipment Associates, Inc.
Electronics Corporation of America
Infra—Red Industrial Systems Div.
Intertech Corporation
Leeds & Northrup Co.
Mac Leod & Stewart Co., Inc.
Nebetco Engineering
Photobell Co., Inc.
Photomation, Inc.
Reliance Instrument Manufacturing Corp.
Sentry Controls, Inc.
Robert M. Wager Company, Inc.
Many of the early publications on smoke density meters such as the papers by
Littlewood, 95 ° Storey, 963 Lambie, 85 ° Collins and Steele, 851 and Kolbow and Thieme 193
deal with details of instrument design. A tungsten light source and a photocell
detector, such as a cadmium sulfide cell, are generally used. However, an instrument
described by licks 1033 uses a bolometer detector.
4wd
p.
THERMO-SYSTEMS INC.
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Since in these instruments a light beam must be projected through windows
on the sides of the stack, these windows must be kept clean to. Lnsure reliable
operation. According to Croase, et al, 487 some of the early att&ipts at keeping
the windows clean by blanketing them with clean air have not been particularly
successful, and they have therefore developed the “Evercican Window”. The
principle of this “Everclean Window” can be seen more clearly in Figure II
presented by Lucas • 13 The air• drawn in from the outside through the small holes
by duct suction keeps the coarse dust from accumulating in the large tube, while
the thin—walled honeycomb, which is cemented to the window, acts as a vapor trap
to keep the vapor from condensing on the window.
projector or
receiver
honeycomb
un it
air drawn in by
duct suction
Fig. 11. The Everclean window described by Crosse, et al,
and by Lucas. 13
487
The relationship betleen the optical density of the aerosol as measured by
a smoke density meter and the mass concentration of the particles is quite complex,
as discussed earlier. The relationship is dependent on the density, shape,
refractive index, and size distribution of the particles. A direct relationship
between optical density and mass concentration can be found only when these
variables remain constant. In general, the optical density is a measure of the
total scattering cross section of the particles. However, in the geometrical
scattering regime the optical density is directly proportional to the total
surface area of the particles, and for a given mass concentration it is inversely
proportional to the mean volume—surface diameter. There have been only a few
experimental studies made on the relationship between the optical density of the
aerosol in stacks and the properties of the particles. 718 , 964 , 485 The results
rain
shield
%gas
\flow
flue
duct
NOT REPRODUCIBLE
ThERMO- SYSTEMS INC.
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—151—
presented by Lucas and Snowsill 485 as shown in Figure 2 appear to be in agree-
ment with the conclusion that, for a given mass concentration, the optical
density is inversely proportional to the mean volume—surface diameter.
Schnitzler et a1 1189 experimentally calibrated two smoke density meters
in terms of particulate mass concentration in a coal—fired effluent duct. They
took 5—minute gravimetric filter samples at the center of the effluent duct and
compared the uieasursments with the corresponding 5—minute average smoke density
measurement. The results are shown in Table 1 and Figures 13 and 14 and the
accompanying discussion of the section entitled “Electrostatic Measurement
Methods”. Tranemissometer A had trouble with window contamination and thus
correlated poorly with niass concentration. Tranamissometer B correlated reason-
ably well with mass concentration on the average. However, both transmissometers
showed different calibration curves for each of the 3 plant operating conditions
in the test. These results show that, although transmissometers may be con-
structed to yield reasonable mass correlation under normal plant operating con-
ditions, the correlation deteriorates during such operating conditions as soot
blowing and partial load. Perhaps most disturbing is that the operator often
does not know whether the mass calibration is valid at a given time.
In order to measure correctly the true transmittance of the aerosol, the
smoke density meter must have pin—hole apertures as shown in Figure 1 to prevent
scattered light from reaching the photocell. Many smoke density meters described
in the literature and available commercially do not have pin—hole apertures, and
they generally accept a substantial amount of scattered light in the forward
direction. The response characteristics of these instruments are much more
complicated. Thus, two instruments of identical design will generally respond
differently to the same aerosol if they are installed in stacks that are different
in size and have different optical path length. Also, the Bouguer, or Beer—
Lambert, law will no longer hold for these instruments. Thus a complicating
factor is introduced which is best avoided by the use of pin—hole apertures.
The question of the desirability of using pin—hole apertures has also arisen
in a related field. In the design of photo sedimentometers for measuring the size
distribution of powder particles by sedimentation and by light extinction measure-
ment, the question has arisen as to whether one should use a narrow—angle instru-
ment, in which pin—hole apertures are used to limit the amount of scattered light
received by the photocell, or a wide—angle instrument, in which the photocell is
deliberately placed very close to the liquid suspension so that practically all
the scattered light due to diffraction around the particles is rece yçd by the
photocell. The question is somewhat controversial, with Hodkinson ’ favoring
a narrow—angle instrument and Allen 34 favoring a wide—angle instrument. The
advantage of the wide—angle instrument, according to Allen, is that the instrument
will operate in the regime of geometrical scattering over a wider range of particle
sizes than the narrow—angle instrument. From the standpoint of instrument design
for stack monitoring, aside from the question of relative merit of the wide—angle
r%lrDIwCVCT 1C i ir
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—.I)L—
and the narrow—angle instrimients, it is difficult to conceive how a truly wide—
angle smoke density meter can be built, since the aperture size in a wide—angle
instrument is of the same order of magnitude as the thickness of the medium.
Thus, the photocell or the collecting lens aperture must be as ‘large as the
thickness of the aerosol illuminated by the light beam and this would appear to
be prohibitively large for an instrument designed for aerosol monitoring in
stacks. The same question of aperture size is considered from a somewhat
different point of view in the section on Angular Light Scattering.
Crosse, et al, 637 and Lucas and Snowsill 485 described an instrument to
measure and record the likelihood that dust emitted from a chimney would cause
nuisance and complaint at ground level. Since only large particles are involved
in causing such nuisance and complaint, they designed an instrument to measure
these large particles. The large particles in the stack gas enter a small opening
on the side of a vertical tube by inertia and settle to the bottom of the tube.
The tube is closed at the upper and lower ends by two glass plates and the amount
of dust settled to the bottom glass plate is measured by light extinction. The
instrument response is rather complicated, being dependent not only on the optical
properties of the particles but also on the somewhat uncertain characteristics of
the particle collection device.
CONCLUSIONS
Smoke density meters based upon the principle of light transmission or
extincation are likely to continue as an important method for the continuous
monitoring of aerosols in stacks. A properly designed smoke density meter with
pin—hole apertures will provide a measure of the turbidity of the aerosol which
is equal to the total light extinction cross section of the particles. In the
geometrical scattering regime, the turbidity is also proportional to the total
surface area of the particles. Thus, in the geometrical scattering regime the
instrument provides a measure of the aerosol mass concentration only when the
volume—surface diameter and the density of the aerosol remain constant and, for
a given mass concentration, the aerosol turbidity is inversely proportional to
the mean volume—surface diameter. The section of this report entitled “Stack
Emissions Properties and Instrument Specifications” shows that particle size
distribution and specific density do not remain constant in fossil fuel effluent
streams, making correlation of mass concentration with light extinction impossible.
From a study of the light scattering properties of small particles, it is con-
cluded that, for aerosol measurement in stacks, a smoke density meter using a
tungsten light source will operate essentially In the geometrical scattering
regime when the particles are larger than 0.5 to 2 i im diameter, the exact limit
being dependent upon the design of the instrument as well as the properties of
the particles.
REFERENCES
26 Adams, J. M., “Light Extinction Photometer for Measurement of Particle
Sizes In Polydispersions”, Review of Scientific Instruments , V. 39,
no. 11, p. 1748—1751 (Nov 1968).
THERMO- SYSTEMS INC.
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—153—
33 Allen, T., “Determination of the Size Distribution and Specific Surface
of Fine Powders by Photo—extinction Methods. I. Theoretical Estimate of
Variation in Extinction Coefficient with Particle Size Using a White
Light Source”, Powder Technology , V. 2, p. 133—140 (1968—69).
34 Allen, T., “Determination of the Size Distribution and Specific Surface
of Fine Powders by Photo—extinction Methods. II. Comparison between
Wide—Angle and Narrow—Angle Photosedimentometers and Experimental
Determination of Extinction Coefficients”, Powder Technology , V. 2,
p. 141—153 (1968—69).
106 Anon, “Particle Size Determination Simplified”, Chemical & Eng. News ,
(Feb 1969).
1212 Born, M., and Wolf, E., Principles of Optics , Pergamon Press (1964).
851 Collins, K. B., and Steele, D. J., “High—Sensitivity Recording Optical
Density Meter”, Journal of Scientific Instruments , V. 38, p. 186—190
(May 1961).
487 Crosse, P. A. E., Lucas, D. H., and Snowsill, W. L., “Design of an
“Everclean Window” for the Observation of the Optical Density of
Flue Gas”, Journal Inst. of Fuel , V. 34, no. 250, p. 503—505 (1961).
637 Crosse, P. A. E., Lucas, D III., and Snowsill, W. L.,”Instrument for
Recording the Dust Nuisance by Chimneys”, Journal of Scientific
Instruments , V. 38, p. 12—17 (Jan 1961).
46 Harris, G. W., “A Laboratory Instrument for Measuring the Turbidity
of a Suspension of Dust Particles”, Powder Technology , V. 3, p. 107—ill,
(1969—70).
1033 Hicks, C. E., “Sensor Measures Stack Effluents—Accurately”, Petro/Chein.
Engr. , V. 39, no. 5, p. 33—34 (1967).
1213 Hodkinson, J. R., “The Physical Basis of Dust Measurement by Light
Scattering”, Aerosols: Physical Chemistry and Application , p. 181—194,
Publishing House of the Czechoslovak Academy of Sciences, K. Spurny,
Ed. (1965).
1211 Hodkinson, J. R., “The Optical Measurement of Aerosols”, in Aerosol
Science , C. N. Davies (Ed), Chap. 10, Academic Press (1966).
718 Hurley, T. F., and Bailey, D. L. R., “The Correlation of Optical Density
with the Concentration and Composition of the Smoke Emitted from a
Lancashire Boiler”, Journal Inst. of Fuel , V. 31, p. 534—550 (Dec 1958).
1214 Kerker, M. Electromagnetic Scattering , Pergamon Press (1963).
THERMO-SYSTEMS INC.
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—154—
1215 Kerker, M., The Scattering of Light and Other Electromagnetic Radiation ,
Academic Press (1969).
193 Kolbow, P., and Thieme, K., “Smoke—Density Meter for Supervising the
Percentage of Dust Contained in the Flue Gases of Industrial Plants”,
Siemens Rev. , V. 38, no. 8, p. 276—278 (Aug 1965).
843 Kratohvil, J. P., “Light Scattering”, Analytical Chemistry , V. 36,
no. 5, p. 458R—472R (Apr 1964).
850 Lainbie, R., “Improved Smoke Density Recorder”, Journal of Scientific
Instruments , V. 37, P. 144—146 (Apr 1960).
950 Littlewood, A., “Measurement of the Optical Density of Smoke in a
Chimney”, Journal of Scientific Instruments , V. 33, p. 495—499 (Dec 1956).
485 Lucas, D. H., and Snowsill, W. L., “Some Developments in Dust Pollution
Measurement”, Atmospheric Environment , V. 1, p. 619—636 (1967).
13 Lucas, D. H., “I. Air Pollution Measurements”, Phil. Trans. Royal
Society of London , V. A257, p. 143—151 (1969).
147 Mie, G., “Beitrage zur Optik truber medien, speziell kolloidaler
Metallosungen”, Annalen der Physik , V. 25, no. 3, p. 377—445 (1908).
754 Musgrave, J. R., and Harner, H. R., “Turbimetric Particle Size Analysis”,
Eagle Picher Research Laboratories, Joplin,Mo., Research Technique &
Technology, no. 1, 47 p. (1947).
1199 Newton, R; G., Scattering Theory of Waves and Particles , McGraw Hill
(1966).
895 Orr, Clyde, Jr., “Automatic Particle Size Analysis in the Subsieve
Range”, Paper 6—1, U. S. — Japan Particulate Technology Seminar, Kyoto,
Japan (6—11 Oct 1969).
846 Penndorf, K. B., “Total Mie Scattering Coefficients for Spherical
Partic1es of Refractive Index n2”, Journal of the Optical Society of
America , V. 46, p. 1001 (Nov 1956).
848 Penndorf,R. B., “New Tables of Total Mie Scattering Coefficients for
Spherical Particles of Real Refractive Indexes (1.33
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—155—
1200 Penndorf, R. B., “New Tables of Mie Scattering Functions for Spherical
Particles, Part 6: Total Mie Scattering Coefficients for Real Refractive
Indexes”, Geophysical Research Paper No. 45 , Air Force Cambridge Research
Center, Bedford, Mass. (1957).
1183. Penndorf, R. B., “An Approximation Method to the Mie Theory for Collodial
Spheres”, J. Phys. Chem. , V. 62, p. 1537—1542 (Dec 1958).
1189 Schnitzler, H., Maier, 0., and Jander, K., “Meastand fur die Prufung and
Kalibrierung von Registrierenden Staub — und Gasmesageraten in einem
Steinkohlengefeuerten Kraftwerk”, SchrReihe Ver. Wass. — Boden Lufthyg.
Berlin—Dahlem , V. 33, Stuttgart (1970).
964 Stoecker, W. F., “Smoke—Density Measurement. Correlation of Solids
Content in Gas with Photoelectric Smoke—Meter Readings”, Mechanical
Engineering , V. 72, no. 10, p. 793—798 (1950).
963 Storey, R. M., “Smoke Density Integrator”, Journal of Applied Physics ,
V. 11, p. 509—512 (Nov 1960).
1250 Stratton, 3. A., Electromagnetic Theory , McGraw Hill (1941).
973 Traxier, R. N., and Baom, L. A. H., “Measurement of Particle Size
Distribution by Optical Methods”, American Society for Testing Materials,
V. 35, p. 457—470 (1935).
1201 Van de Huist, H. C., Light Scattering by Small Particles , Wiley (1957).
THERMO- SYSTEMS INC.
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MULTI-WAVELENGTH LIGHT TRANSMISSION
by: B.Y.}1. Liu and K. T. Whitby
INTEODUCTION
The transmission of light or other electromagnetic radiation through a
particulate suspension is dependent upon the wavelength of light used. This
fact can be utilized for the measurement of particulate suspensions, including
aerosols. By observing the change in the attenuation of light through a
particulate suspension with wavelength, certain physical properties of the
suspension can be inferred.
Multi—wavelength light transmission, or spectral attenuation, measurement
is a powerful tool in the study of colloids and aerosols, where few other methods
are as simple and as easy to use. Further, within the limits of its useful size
range, the method can provide a measure of the volumetric concentration and the
size distribution of the particulate phase. Thus, if the particle density is
known or remains fixed, the aerosol mass concentration can be determined.
The transmittance of an aerosol can be measured by means of a transmissometer,
which is dicussed in the chapter entitled “Light Transmission”. Therefore, the
details are not repeated here. In order to reduce the amount of scattered light
reaching the optical sensor and to measure the true transmittance, the angular
apertures of the instrument must be limited. Because of the reduced light
intensity due to the use of monochromatic light, a photomultiplier, instead of
a photocell, may have to be used.
To measure the transmittance as a function of wavelength, the wavelength
must be varied. This can be accomplished either with monochromatic filters or
with a monochromator. In principle, one can also use a tungsten source and vary
its temperature to vary the mean effective wavelength, but this approach does not
appear to have been tried.
BASIC PRINCIPLE
There are three possible methods whereby the same general principle can be
utilized for size and concentration measurements. These are usually referred to
as the first maximum method, the dispersion quotient and turbidity ratio method,
and the wavelength exponent method. These methods have been reviewed and discussed
in some detail in the recent book by Kerker 1215 (pp. 334—343).
ThERMO - SYSTEMS INC.
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—157—
The general principle underlying these methods can be seen by referring to
Figure 1. This figure shows the typical relationship between the particle
extinction coefficient K(m,c&) and the dimensionless particle size parameter
U ltD/A
(1)
calculated from the Mie theory. Here m is the refractive index of the particle,
D is the particle diameter (assuming a sphere) and K, the particle extinction
coefficient, is defined as the ratio of the flux scattered and absorbed by the
particle to the flux geometrically incident upon the particle. It is seen that
for particles of a specific size varying the wavelength, A, varies the dimension-
less size parameter, a, and causes a similar change in the extinction coefficient,
K. The corresponding change in light attenuation through the suspension is the
basis of the methods considered in this section.
The first—maximum method was first applied to monodisperse particulate
suspensions by La Mer and Sinclair 1203 and later used by many others for measuring
the size and the concentration of particulate suspeusionsll 86 ,ll 82 ,ll72,1202
In applying this method to monodisperse systems, the dimensionless particle size
parameter, a, is varied by varying the wavelength until it reaches a point
corresponding to the first maximum in the particle extinction curve. At this point,
the turbidity, k, of the suspension, which is related to the experimentally measured
transmittance, T, as follows
T exp (— k £),
(2)
4 O
10
Fig. 1. Typical particle extinction curve.
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—158—
should also show a maximum. Here, £ is the path length of the light beam
through the suspension. Knowing the value of at the first maximum from
theory and the wavelength A at which the maximum is observed, the particle
diameter can then be determined by means of equation (1). To calculate the
concentration of the particles in the suspension, the following relationship
between turbidity, k, and concentration is used:
k -(1/4) irD 2 K (3)
where C is the number concentration of the particles. Since the volumetric
concent ation of the particles, C , 7 , is related to the number concentration
C , as
C = /6) D 3 C , (4)
equation (3) is equivalent to
3K C
k= 2D (5)
Thus, knowing the value of K from theory and the value of k and D from experi-
ment, the volumetric concentration C, can be calculated by means of equation (5).
For a polydisperse syBtem of spherical particles, equations (3) and (4) must
be integrated over the appropriate size distribution. Dobbin8 and Jizmagian 6 O 3
have shown that equation_(5) continues to apply provided K is replaced by the mean
extinction coefficient, K, defined as
2
- S D 2 f(D) dD
and D is replaced by the volume—surface, or the mean Sauter, diameter, D , defined
as vs
D = I D 3 f(D) dD
vs 2 (7)
5D f(D) dD
Thus, for a polydisperse system,
3
2D (8)
vs
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—159—
Dobbins and Jizinagian have also shown that the values of i obtained by integrating
over a rectangular, a parabolic and an upper limit distribution function are
essentially the same and that there is a universal function relating K and the
phase shift parameter
2(rn — l)cx (9)
where the dimensionless particle size parameter, is based on the volume—surface
or mean Sauter diameter of the suspension. Table 1, reEroduced from the paper
of Dobbins and Jizrnagian, shows the relationship between K and a with the
refractive index, rn, as a parameter. On the basis of these resuY!s it can be
concluded that the first maximum method should continue to apply to polydisperse
systems.
Table 1
Data from Dobbins & Jizmagian 603 showing the mean
particle extinction coefficient K as a function of
the phase—shift parameter, p 2 (m — l)cz with the
refractive index, rn of the rtic] .e as a pJameter.
‘H
1.20 1.33 1.40 1.50 I 05 1.73 I $5
(0.o 40). 0.04924 0.03601 0.02336 001297 0 00940 I)A 10658
1.0 40. 3 ( k) , (4.39146 0.J O0 0.2142 4 0.1957 (LI 504 (LI I SO
1 5 (1.122) 1 043 o. ’$187 0.’1 176 0.761414 0.65144 0.54’)5
I S lO 1.7141 1.7614 1 726 I.(i21 1.523 1.112
2 4’)’) 2.146 2.469 2 $79 2.175 2.4fl 2332
40 . 21424 2.951 2.9146 .1.011 3.150 3.1.15 3.0144
.o37 4 2 1$ .4 275 3.M J 3.5o2 .$.S’) O 2 coo
I I ) 4114 4.272 .435 0 .4.1514 4717 .4.775 . 477o
a 2.’I al .4000 .4.147 3.249 .4.521 .4. Oo S
Il l ) 2.01$ 277.4 2.1442 2.930 3.1i2 .4.23.4 1274
2.3 $’) 2A4 ’ ) 2.725 2.’rn& 2.97$ .4021
4 ) 2 .4 ;’) 2. I’k 2.544 2.642 2.7Wi 2.14 1 2 S14I
“4 ) 2 20. ; 2.127 2476 2.5414 2 74 2.739 2.7147
114.0 2 2S 2 .414.4 2 42$ 2.4146 2(110 2074) 2.7114
2.24k) 2327 2.368 2.420 2.c22 Lc77 2624
44.4) 2.114 1 2.214 2 327 2.3(1.5 Ll 2 3111 2.553
l6 J 245’) 2.255 2.2’)6 2.330 2.112 2.457 2.497
1140 2 142 2.34k) 2270 2.302 2.37.4 2.414 2.452
u.o 2.12’) 2.20$ 2.246 2.279 2.444 2.3714 2.413
400 2.0147 2.147 2.168 2.195 2.234 2.262 2.2W)
2042 2.0447 2.099 2.117 2.173 2.47’) 2.182
(flU 2.1)24 2.042 2.049 2.059 2.075 2.0143 23)93
Values U I l !UUhIIC 5U 5 , ,bi. ,u ,w ,I by i tr.iid itmn.
i1PRODUCIBLE
Th RMn.cvcTFMs I 4r
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The question-of how the first maximum in the particle extinction curve is
affected by the polydispersity of the suspension was also studied by Hulbig. 905
The author used a log—normal and a Maxwellian distribution, and described a
procedure for determining the parameters in these distributions from a knowledge
of the volumetric concentration of the suspension and the location of the first
maximum as determined by spectral attenuation measurements.
The first maxi tjm method was applied to irregularly shaped particles by
De vore and Pflund)•° 5 The authors measured the spectral attenuation of
particulate suspensions consisting of uniform sized but irregularly shaped
particles over the wavelength range from 0.8 to 3.0 pm and found typical turbidity
maxima similar to those observed for spherical particles. The measured particle
sizes based on the first maximum method were in excellent agreement with those
found by light microscopy.
Since the particle extinction coefficient is also a function of the refractive
index of the particle, to use the first maximum method for size and volumetric
concentration measurements the particle refractive index must also be known.
Additionally, to measure the mass concentration of the suspension, the particle
density must be known. Both of these parameters,if unknown, presumably can be -
determined by an independent procedure. Thus, in actual practice, it is only
necessary that these parameters remain unchanged.
The particle size range of the first—maximum method is dependent upon the
refractive index of the particles and the range of wavelength that is available.
From Table 1 we see the first maximum in the particle extinction curve occurs at
approximately the point Pvs = 2 a(m — 1)’ = 4. For example, if m = 2, then a
= ltD/A = 2. Thus if A can be varied within the range from 0.35 to 2 im, theX 5
the corresponding range in Dy 5 is from 0.22 to 1.27 pm. Thus, the size range of
the method is quite limited. However, within its useful size range the method
does provide a measure of the particle mass.
The turbidity ratio method and the dispersion quotient method are similar in
principle and both require that the turbidity, It, of the suspension be measured
at two preselected wavelengths, A 1 and A 2 . Figure 2 shows the turbidity ratio as
a function of the particle size calculated from Mie theory for a particle refractive
index of 1.2 and A 1 = 0.365 and A 2 = 1.01 pm. Using this method, Dobbins and
Jizmagian 602 measured the volume—surface mean diameters and the volumetric con-
centrations of several latex suspensions. The results are in satisfactory agreement
with the corresponding values measured by electron microscopy and by a gravimetric
method, the maximum error in the volume—surface mean diameter and the volumetric
concentration being on the order of 10% and 13% respectively. The same method was
also applied by Dobbins 57 ’ 561 to the measurement of aluminum oxide particles in
the exhaust of a small rocket motor.
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Fig. 2. Turbidity rat o for m — 1.2 and A 1 — 0.365 and
A 2 — 1.01 02
In the dispersion quotient method, first suggested by Teorell nearly 40
years ago, the dispersion coefficient defined as
2
k 1 X 2
k 2 A 1
(10)
is determined experimentally by measuring k 1 and k 2 at the wavelengths A 1 and A 2 .
By referring to a theoretical curve for the dispersion quotient as a function
of particle size, the particle size is determined. Figure 3 shows the comparison
of the theoretical and the experimental dispersion coefficients for m — 1.214 and
1 ,200 andA 1 —0.4O5andA 2 —O546,
Fig. 3. Theoretical and experimental dispersion coefficiént. 85
0, tv
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The wavelength exponent method was a p1ied to polystyrene latex suspensions
by Reller, et al, 1 - 44 and Bateman et al, ’ 1- and studied in considerable detail
theoretically by Heller, et ai.,1 1 84 and by Reller. 118 The method is based on
the observation that for a specific suspension, if the variation of the turbidity,
k, with the wavelength is written as follows,
k (const)A (11)
the wavelength exponent, g, is a function of the dimensionless particle size
parameter, a. The exponent, g, is equal to 4 for Rayleigh scattering and decreases
steadily to zero with increasing particle size or decreasing wavelength and
eventually goes negative. Thus, knowing the wavelength exponent, g, and the wave-
length, the particle size can be determined. The wavelength exponent, g, is
usually determined by measuring the turbidity at two slightly different wavelengths.
Figure 4 shows the wavelength exponent calculated from Mie theory.
Fig. 4. The wavelength exponent, g, for a slightly polydisperse
system. 1184
The size distribution of particulate suspensions can sometimes be inferred
from spectral attenuation measurements. The process of deriving the sizedistri—
bution from spectral attenuation measurements is a complicated one and many
procedures have been devised. The procedures generally fall into two classes. In
one, the spectral attenuation curves are calculated for various- size distribution
functions. The curve that appears to fit the experimental data the best is selected
and the corresponding distribution function is taken to represent that of the
suspension. In the second procedure, the basic equation for calculating the
turbidity
k (A) = 5D 2 K(A) C f(D) dD (12)
9
a
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—16 3—
is regarded as an integral equation and solved numerically on a computer to
obtain the size distribution. These procedures have been discussed in considerable
detail by Kerker’ 215 and by Yrnnnn oto and Tanaka. 22 We shall not discuss them in
further detail here since the possibility of applying them to general stack
monitoring appears to be quite limited.
For making spectral attenuation measurements, various investigators have
generally used the general purpose laboratory spectrophotometer, for which there
are many manufacturers. However, an instrument specially designed for measuring
flue gases is available from the DuPont Co. (Model 400 photometric analyzer,
Instrument and Equipment Division, Wilmington, Delaware 19898). In this instrument
the wavelength can be varied between the limits of 210 and 1000 nm.
APPLICATION TO MASS CONCENTRATION AND SIZE DISTRIBUTION MEASUREMENTS IN STACKS
Spectral attenuation measurement can be considered as a natural extension
of the usual smoke density meter in which white light from a tungsten source is
generally used. As such, it possesses all the advantages of the smoke density
meter including the relative simplicity and the relative 1 .0w cost of the instrument
as well as the ability to integrate the reading along the path of the light beam.
The fact that the instrument does not have to be in actual physical contact with
the flue gas, except for the window through which to project the light beam, is
of course, another advantage. The added requirement of changing the wavelength
does not necessarily mean that the instrument is considerably more complex than
the usual smoke density meter, since the wavelength can be simply changed by
turning a wheel on which a series of filters has been mounted.
Any one of the three methods discussed in the preceeding section can be used
to measure the mean volume—surface diameter of the particles and the mass con-
centration of the aerosol. The equipment required is essentially the same, the
difference being primarily in the method of data interpretation and reduction. In
order for the instrument to measure the true aerosol mass, the refractive index
and the density of the particles must remain constant with time so that, once
the values of these parameters are determined for a specific emission source, they
can be used subsequently for the same source and similar sources with similar
particle characteristics. This requirement may turn out to be a major limiting
factor In the application of these methods. Unfortunately, the size ranges of
these methods are quite limited. The mean volume—surface diameter of the aerosol
covered by this method extends at most over a decade, because of the limit in the
wavelength range that is available for such measurements. However, if the technical
problem of making the measurement, say up to a wavelength of 5 pm, can be solved,
then upper size limit can be extended considerably beyond 2 pm which we regard as
the upper limit based on existing technology. Some further research on the
application of these methods to stack monitoring definitely appears worthwhile.
Even if the upper size limit cannot be pushed further, the method as it exists
should be useful for such sources as oil—fired furnaces and incinerators where
the mean particle size may be small and be within the range of these methods.
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Although in principle it should be possible to infer the particle size
distribution from spectral attenuation measurements, the data reduction process
does not lend itself to the type of automatic operation required for stack
monitoring. In addition, the method works the best only on relatively narrow
size distributions and the variation in these narrow size distributions that can
be detected by the method appears to be of little interest from the standpoint of
stack monitoring.
REFERENCES
1186 Barnes, M. D., and LaMer, V. K., “Monodispersed Hydrophobic Colloidal
Dispersions and Light Scattering Properties, II. Total Scattering from
Transmittance as a Basis for the Calculation of Particle Size and
Concentration”, J. Coil. Sci. , V. 1, p. 79—91 (1946).
1172 Bateman, J. B., Weneck, E. J., and Eshler, D. C., “Determination of
Particle Size and Concentration from Spectrophotometric Transmission:,
3. CoIl. Sci. , V. 14, p. 308—329 (Jun 1959).
1255 DeVore, J. K., and Pflund, A. H., “Optical Scattering by Dielectric
Powders of Uniform Particle Size”, 1. Optical Society America , V. 56,
p. 1351 (1947).
1185 Dezelic, G., Dezelic, N., and Tezak, B., “A Simple Method for Particle
Size Determination by Turbidity Measurement”, 3. Colt. Sci. , V. 18,
p. 888—892 (1963).
601 Bobbins, K. A., “Measurement of Mean Particle Size in a Gas—Particle
Flow”, AIM Journal , V. 1, p. 1940—1942 (1963).
757 Bobbins, K. A., “An Emission—Scattering Photometer for Particle Size
Measurement”, Brown University, Providence, K. I., Report on Research
for Jet Propulsion Lab., Padadena, Calif., Contract No. JPL 950573,
(Aug 1964).
603 Bobbins, K. A., and Jizmagiau, G., “Optical Scattering Cross Sections for
Polydispersions of Dielectric Spheres”, Optical Society of America
Journal , V. 56, p. 1345 (1966).
602 Bobbins, R. A., and Jizmagian, G., “Particle Size Meaaurements Based on
Use of Mean Scattering Cross Sections”, Optical Society of America
Journal , V. 56, p. 1351 (1966).
567 Bobbins, K. A., “Remote Size Measurements of Particulate Products of
Heterogeneous Combustion”, 11th International Symp. on Combustion,
Combustion Inst., Pittsburgh, Pa. (1967).
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759 Dobbins, R. A., and Strand, L. D., “A Comparison of Two Methods of
Measuring Particles Size of A1 2 O 3 produced by a Small Rocket Motor”,
Brown University, Providence, R.I.
1184 Heller, W., Bhatnagar, H. L., and Nakagaki, M., “Theoretical Investi-
gations on the Light—Scattering of Spheres. XIII. The Wavelength Exponent
of Differential Turbidity Spectra”, J. Chem. Phys. , V. 36, p. 1163—1170
(1962).
1183 Heller, W., “Theoretical Investigations on the Light Scattering of
Spheres. XV. The Wavelength Exponents f Small c& Values”, J. Chetn. Phys. ,
V. 40, P. 2700—2705 (1964).
905 Hulbig, C., “Particle Size Distributions of Polydisperse Systems by
Means of the First Maximum Methods”, Staub—Reinhalt der Luft (Engi. Trans.) ,
V. 29, no. 5, p. 1—5 (May 1969).
1211 Hodkinson, J. R., “The Optical Measurement of Aerosols”, in Aerosol Science ,
C. N. Davies (Ed.), Chap. 10, Academic Press (1966).
1182 Johnson, I., and LaMer, V. K., “The Determination of the Particle Size
of Monodispersed Systems by the Scattering of Light”, Amer. Chem. Soc. J. ,
V. 69, p. 1184—1192 (1947).
1202 Kenyon, A. S., “Higher Order Tyndall Spectra in Monodispersed Sulfur
Sole”, Trans. N.Y. Acad. Sd . (1962).
1215 Kerker, M., The Scattering of Light and Other Electromagnetic Radiation ,
Academic Press (1969).
1203 LaMer, V.K., and Sinclair, D., “Verification of the Mie Theory”, OSRD
Rep. No. 1857 and 944, Office of Pubi. Board, U. S. Dept. of Comm.,
Washington, D. C. (1943).
22 Yamamoto, G., and Tanaka, M., “Determination of Aerosol Size Distribution
from Spectral Attenuation Measurements”, Applied Optics , V. 8, No. 2
p. 447—454 (Feb 1969).
Reviewed But Not Cited in Test
1204 Cole, J. E., III, Dobbins, R. A., and Seinerjian, H., “Time—Resolved
Measurement of Droplet Size and Concentration in Cloud Chambers”, to
appear in J. Appl. Met . (1970).
792 Conner, W. D., and Hodklnson, J. K., “Optical Properties and Visual
Effects of Smoke—Stack Plumes”, National Center for Air Pollution Control,
Cincinnati, Ohio, Clearinghouse No. PB 174 705 (1967).
836 Hodkinson, J. R., “Some Observations on Light Extinction by Spherical
Particles”, British Journal of Applied Physics , V. 14, p. 931—932
(Dec 1963).
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735 Lanzo, C. D., and Ragsdale, R. G., “Experimental Determination of
Spectra], and Total Tranmnissivities of Clouds of Small Particles”,
Lewis Research Center, Cleveland, Ohio, Clearinghouse No. N 62 16012
(Sep 1962).
631. Marteney, P. J., “Experimental Investigations of the Opacity of Small
Particles”, United Aircraft Corp., Zast Hartford, Conn., Clearinghouse
No. N 65 20983 (1965).
170 Takahashi, K., “Determination of Number Concentration of Po].ydisperaed
Small Aerosol Particles by Turbidity Measurement”, Journal of Colloid &
Interface Science , V. 24, no. 2, p. 1,59—163 (1967).
137 Wallach, M. L., He].Ler, W., and Stevenson, A. F., “Theoretical Investi-
gations on the Light Scattering of Colloidal Spheres. X II: The Deter-
mination of Size Distribution Curves from Turbidity Spectra”, Journal of
Chemical Physics , V. 34, p. 1796—1802 (196]).
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INTRODUCTION
LIGHT SCATTERING: POLARIZATION RATIO METHODS
by: N. Baraic, B.Y.H. Liu and K. T. Whitby
When unpolarized incident light is scattered by an object, the scattered
light intensity can be described by two plane polarized components, i 1 , perpendi-
cular to, and i 2 , parallel to the plane of observation. There is a relationship
between the intensity of scattered light and the wavelength of incident light,
object size, shape, size distribution, index of refraction, and observation angle.
Most light scattering studies utilize the sum of the two intensity components to
determine the particle size and distribution of an aerosol, as in the conventional
optical counter and aerosol photometer. However, in the polarization ratio
method considered in this section, the ratio, i 1 /i 2 , is used as a measure of the
particle size.
BASIC PRINCIPLE
If the light scattering theory is applied to a monodisperse aerosol. system,
a unique relationship is found to exist between the polarization ratio, i 1 /i 2 ,
and particle size for right angle scattering with ct < 2.5. Here ct nd/A is the
dimensionless particle size parameter that occurs in light scattering theories,
d is the particle diameter and A is the wavelength. The device commonly used to
perform these experiments is illustrated in Figure 1. The eyepiece is fitted with
a split field polarizer disk, one half having its direction of vibration parallel to
Fig. 1. Instrument for determining angular ribution of spectral
colours and polarization — the Owl.
ThERMO- SYSTEMS INC.
lyr ._Wft/F. f./Zvo/t lamp in
odjustot e fr .wd r
L:r .?; . bcWzd so thaI 0
pcic : it om a ’/# tht is
a m& by ,7lumin tor
Jmch scale
and exit va4’es
Thlzscope
Tekscope is focussed
on cQntre o cell
frQp
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—168—
a dividing line and the other halt perpendicular to it. The dividing line is
usuaiiy parallel to the plane of observation. A filter is placed in the optical
system between the polarizer and the observer to yield nearly monochromatic light.
The polarizer is rotated until both halves of the field are equal in brightness.
This angle, $, is related to the polarization ratio by the following equation:
tan 2 •— • p(e)
The system can then be calibrated with different sized aerosols and indices
of refraction. The data are s umnarized in a series of curves as shown by Figure 2
from Green and Lane. ’ 216
Rod/us in microns
Fig. 2. Analyzer angle • as a function of radius of particle. 1216
Because the measurement obtained by this method is a ratio, no instrument 144
constant is necessary, which greatly simplifies the application of the method.
0 00S 0/0 0 Oa?0 OZS OJO 0 5
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The polarization ratio method is easily applicable to studying both
monodisperse and polydisperse aerosols. The polarization ratio increases
monotonically from its vaiRe of cos 2 0 for small particles up to a maximum
atapproximately a 2.5.907 If some knowledge about the aerosol system is
available, a single measurement of p(0) at one angle can provide a measure
of particle size for a monodisperse aerosol. For polydisperse aerosols the
polarization ratio is defined in terms o a size distribution function as
presented in the equation below:
Jf (a)i (6,a)da
p(o) — I (0)/I (0)
f f (a)i 1 (0,ci)da
where
f(a) is the size distribution function
0 is the scattering angle
a is ird/X
d is particle size
A is light wavelength.
Analysis of an aerosol is then reduced to determining the distribution
function corresponding to measured values of p(6) by comparison with
theoretical calculations. Theoretic 2 values of p(0) have berg computed for
many cases. 1215 lieller and Wallach’ and Takahasbi and Iwal present detailed
approaches to this problem. The exact procedure is complex and time consuming;
therefore, initial studies indicate that this method is appropriate for research
applications only.
The effect of varying the spread (increasing the standard deviation, a)
of the distribution function is illustrated in Figure 3.
Th ERMO- SYSTEMS iNC.
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Fig. 3. Plot of p(O) versus 9 for & 2.0 and a
0.145, 0.200, 0.250, and 0.300. The insert shows
extreme distribution curves, a radius in nw. 7 1
0.100, 0.125,
the corresponding
Figures 3 and 4 illustrate the increase in oscillation evident as
increases. Also obvious in these two figures is the tendency of the oscillation
emplitude to decrease as a (standard devivation) increases. These graphs
indicate that a more precise determination of size distribution is possible for
narrower distributions, and that a — 0.3 is the upper limit of this method.
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Note that as a changes, the extreme of these curves remains at the
same angular position. This tendency provided an earl .r met çd of determin.
ing particle size using single particle scattering functions. ‘
Fig. 4. Plot of p(6) versus 0 for 5.0 and a 0.100, 0.125,
0.145, 0.200, 0.250 and 0.300. The insert shows the corresponding
extreme distribution curves, a — radius in mii. 71
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The effect of refractive index on polarizat gn ratio measurements is
illustrated in Figure 5 from Takahashi and Iwai. This figure presents
the dissyimnetry factor (a form of polarization ratio) as a function of log a
for several values of a and refractive index. g
Although the polarization ratio method has not been treated as
1
Fig. 5. Diasyumietry factor vs. particle size distribution:
(A) m 1.33, (B) m — 1.44, and (C) rn — 1.55.
A
B
C
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extensively as direct light intensity measurements, it does offer advantages
provided the size parameter, c , is less than 2.5, the size distribution standard
deviation is less than 0.3 and refractive index is relatively uniform. Within
these limits, polarization ratio measurements provide a useful research tool
for determining the size distribution of an aerosol.
APPLICATION TO MASS EMISSIONS MEASUREMENT IN STACKS
Measurements of the polarization ratio of scattered light intensity is
useful to determine mass concentration only if aerosol characteristics such
as density and particle shape are available. Such information is necessary
because this method; like all optical techniques, is sensitive to particle
geometry and not mass; therefore direct mass concentration data cannot be obtained.
REFERENCES
1216 Green, H. L., and Lane, W. R., Particulate Clouds: Dusts, Smokes and
Mists , E. and F. N. Spon Ltd., London (1964).
144 Heller, W., and Wallach, M. L., “Experimental Investigations on the
Light Scattering of Colloidal Spheres. V: Determination of Size
Distribution Curves by Means of Spectra of the Scattering Ratio”,
Journal of Physical Chemistry , V. 67, p. 2577—2583 (1963).
1211 Hodkinson, J. R., “The Optical Measurement of Aerosols”, in
Aerosol Science , C. N. Davies (Ed.) Chapter 10, Academic Press (1966).
1215 Kerker, M., The Scattering of Light and Other Electromagnetic Radiation ,
Academic Press (1969).
71 Kerker, M., Matijevic, E., Espenscheid, W. F., Farone, W. A., and
Kitani, S., “Aerosol Studies by Light Scattering. I. Particle Size
Distribution by Polarization Ratio Method”, Journal of Colloid Science ,
V. 19, p. 213—222 (Mar 1964).
147 Mie, C., “Beitrage zur Optik truber medien, speziell kolloidaler
Metallosungen”, Annalen der Physik , V. 25, no. 3, p. 377—445 (.1908).
580 Ogle, II. M., “Particle Counting Techniques Available for Aerosol Research”,
APCA Journal , V. 18, p. 657—659 (Oct 1968).
140 Stevenson, A. F., Heller,W.,& Wallach, M. L.,”Theoretical Investigations
on the Light Scattering of Colloidal Spheres. XI: Determination of Size
Distribution Curves from Spectra of the Scattering RAtio or from
Depolarization Spectra”, Journal of Chemical Physics, V. 34, p. 1789—1795
(1961).
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909 Sinclair, D. and LaMer, V.K., “Light Scattering as a Measure of
Particle Size in. Mroeols”, Chemical Reviews , ‘1. 44, p. 245-267 (1949).
58 Takahashi, I C, and Lwai, S., “gatimation of Size Distribution of Small
Aerosol Particles by Light—Scatterjng Measurements”, Journal of Colloid
& Interface Science , V. 23,p. 113—119 (1967).
1201 Van de Hulat, H. C., Light Scatterin&by Small Particles , Wiley (1957).
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ANGULAR LIGHT SCATTERING
by: N. Barsic, B.Y.H. Liu and K. T. Whitby
INTRODUCTION
Light scattering refers to the redirection of illumination that is incident
upon an object. Light is scattered by a combination of transmission, reflection,
and diffraction; and depends upon characteristics of the object, the surrounding
medium, and the incident radiation. Important parameters which influence light
scattering are the light wavelength, particle size and shape, the refractive index
of the particle with respect to the medium, and the angle at which an observer is
located in relation to the incident light.
The nature of light scattered at various angles from the direction of illumi-
nation provides an effective means of studying aerosols. The light intensity as
a function of observation angle provides information that describes particle size
if the index of refraction and light wavelength are known and the particle is
spherical. Such information can yield an estimate of particle mass concentration
if particle density is known.
CHARACTERISTICS OF ANGULAR LIGHT SCATTERING
A. Basic Theory
147
Analytical studies by Mie provide the most comprehensive theory of light
scattering. However, simplifications are applicable for particles that are much
larger than the wavelength of incident light (geometric optics) and particles much
smaller than the wavelength of incident light (Rayleigh scattering). The net
result of this theory is a relationship between the intensity of scattered light
as a function of viewing angle, 6, the light wavelength, A, particle diameter, d,
index of refraction, m, and viewing distance, R:
1(6) f (0, A, d, m, R). (1)
A different functional relationship exists for the three major scattering
regions: Rayleigh, Mie, and geometric scattering. The exact nature of the equations
is presented by Van de Hulst, 1201 Hodkinson, 121 Call,et al, 702 ICerker, 12 5 and
Sinclair and La Mer. 909 Solving the scattering equations is usually difficult and
expensive, even with a modern digital computer. Therefore, mathematical simplifi—
c tions for special cases are extremely valuable.
Important simplications are possible if either the refractive index or particle
size parameter ( ird/X) is very close to 0 or . Van de Hulst’ 201 presents a
figure and an accompanying table to st1t ,,inrize the regions of refractive index and
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particle size parameter where particular solutions are applicable (Fig. 1 and
Table I). There are six boundary regions (1. to 6) where one of the parameters
may have an arbitrary value from near 0 to infinity. The applicable computational
methods are indicated in Table I. The corner regions: 61, 12, 23, 34, 45, and 56 are
characterized by even simpler theories as indicated in Table I. The simplest math—
statical reductions and the clearest physical interpretations of scattering theory
are obtained in regions of the m — domain that are far from dividing lines.
Fig. 1. Survey of limiting cases in the m — domain’ 201
Table I. Boundary Regions of the m — douiain ’ 20 ’
Region in—i r(rn —1) Chapter or Section
Extinction Formuin
Cl
a
•
a
Q = (32/27flm — l) iZ1
1
art
a
a
7.2 (Ravle.gh.Cnn )
12
1
a
a
2
1
a
art
11 (anomalous diffraction)
23
S
a
I
3
1
art
1
12 (largo epheres)
34
1
1
1
Q=2
4
art
5
5
10.6 (total reflector)
46
a
5
1
Q=( 10 13)xl
5
a
S
art
10.5 (optical resonance)
55
a
S
•
S
a
art
a
6.3 (Rayleigh ecattering)
1
0 • I
I I
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B. Intensity Diagrams
The net result of calculations outlined by the intensity equations is a
relationship between the resulting light intensity at a given angle for different
values of particle diameter and light wavelength. Most theoretical work completed
thus far concerns spherical particles. For example, the Rayleigh scattering
intensity function is composed of two components: Ii(8) is independent of e and
therefore gives a circular pattern meaning equal scattering in all directions, and
12(0) is proportional to cos 2 0 and therefore gives a figure 8 pattern. These
results are summarized in the upper left of Figure 2 where scattered light intensity
per particle is presented as a function of scattering angle in addition to the
corresponding polar diagram. This polar diagram shape is representative of all
Rayleigh scatterers since the basic form of the intensity equation is a simple
function of sin 2 0.
I
I
I
Fig. 2. Mie—theory scattering of monochromatic light by’
single spheres of refractive index 1.5.1211
The relative angular distribution of scattered intensity and degree of polarization
is completely symmetrical forward and backward and does not vary with particle size
and shape. For small particles, the scattering by moderately absorbing particles
of complex refractive index m(l — i k), k < 1, is nearly the same as that by non—
absorbing particles with the same real component of refractive index.
The scattering diagrams of larger particles might be expected to differ from
particles in the Rayleigh region because the governing intensity functions are
noticeably different. ’O2 This is indeed true and is demonstrated in the two lower
illustrations of Figure 2. In addition, the general shape of Mie region scattering
)-\ I.,
\dO7
—II ———Is
Scotisring — (d)
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patterns varies as a function of particle size, unlike Rayleigh scatterers. For
increasing particle size, the Mie scattering pattern becomes more forward directed
and develops an array of angular maxima and minima in addition to the principal
forward maximum evident in Figure 2. Polar diagrams of scattered light for
parameters of 1, 10, and 30 in Figure 3 clearly illustrate this phenomenon. 1 ’
In general, the number of minima in the pattern between 0 00 and l8O is
approximately .
catt ri. d-ii jht intensity (arbitraiy inits)
Fig. 3. Polar diagrams of scattered light for three different Mie parameters
(refractive index of the particle material n — l.50).1233
102
1
10
102
j 4
108 10 ’
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For spheres that are significantly larger than the wavelength, the diff 1 5 ed
radiation is concentrated very close to the forward direction. Van de Hulat
has considered this situation in detail and determined that the exact scattering
theory can be replaced by geometric ray optics for dielectrics with — 10 to 20
except in the neighborhood of the rainbow angle where it will fail regardless of
the value of ,12l5 A comparison of exact theory and geometric optics is presented
in Figure 4.
Fig. 4. Angular distribution function calculated as the sum of
contributions from diffraction, reflection, and refraction
compared with the exact values (dashed line) for m — 2.105.
A 8ize distribution over the range a — 10 to 15 is assumed 2
for which the frequency of particles is proportional to a
(Hodkinson and Creenleaves, 1963).1215
Since the light intensity observed at different angles is a function of particle
size, light intensity measurements provide a means of determining particle size.
Kerker, et al, 7 ° describes a method of finding particle size data by observing the
minima and maxima in the angular scattering pattern. Intensity measurements are
often made at a fixed angle and compared with theoretical intensity values or 842
experimentally determined values from a standard aerosol. Kratohvil and Smart
describe the calibration of an angular light scattering system using Dow polystyrene
latex spheres as a reference.
Many investigators have calculated values of the scattering function for a
variety of conditions. In the past, investigators first tried to seek tables of
scattering functions that had been compiled previou81y such as the tables presented
by Shahroken and Wolf, 474 and McCormick and Lawrence,bOS and in Table II from
liodkinson. 85 Perhaps the most complete table of such authors is presented by
9
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Rodkinson. 85 Because of the voluminous nature of these tables and the lack of
an exhaustive set of tables, many investigators have found it just as convenient
to calculate the desired intensity functions. The availability of digital computers
makes direct calculation more feasible than table use, especially if additional
calculations are to be performed with intensity values.
Table II. Principal published Mie theory computations of the angular
distribution of scattered intensity for medium and large
transparent and moderately absorbing spheres. 85
Refractive AI,soiivt ion
Itange
avid priv rcvn(vIits of
StiLl Intl hg
Partkle size
.
index
coef liriciit
titigle
parameter
: iitI i o i
?7I
k
0
a 7rrl/X
N. Mi ,ti aiul IL. I ikvitIui, iltill. Klent.i’otech.
1.33
00(50) to IS0°
5.3 (5) to 1(1
L Lh. (T ’I v 21, 561; 22, 209 (1937).
O°(2.5°) to 18(1°
10 to 18.5 (23 values)
1 . ). ( 1sivpii ll , N. L. Sting, J 11. Chin,
-
1.33
—
0°(1 0) to 10°
6, 8, 10(5) to 49
mid C. XI. Iih’IH(tVirll, J. Opt. Soc. Am. 42,
10°(10°) to 180°
226 (1952).
1.. I . Aslilov arid (‘. \E. Colili, J. Opt.. Soc. Am.
1.20
(1°(1°) tv, 10°
1, 2, 3, 5, 8, 10, 15, 20,
48, 261 (1 91s).
1 00( 0°) to 18(1°
:to, s.
M. Iwrker avid K. I:Li ijevié, J. Opt. Soc. Ant.
2. 105
—
0°( 10°) to 180°
0.2(0.4) to 5.8, 6.0(2)
51,87 (1961).
to 15.0
P. II. ( lic ’a ’, Z. A toplvs. 51, I 19 (l96l).
1.33, 1.55
—
O°(2°) to 18(1°
1(1) to 40
J . It. I’eiinilui -f, U. S.Air I’on-c ( ipliysics
1.1, 1 .2, 1 .33
—
()°(5°) Isv 180°
0.1(0.5) to 10
I e earrlt I )ircihiir:ite, i3viilford, Mass.,
1 .4, 1.44, 1.5-
(1961) Tedi. I epI . I AI)—TI(—I;l—;12, (1962)
‘l ’ei -It. I ept. OAI)—T0—63—9, (1963) Tedi.
.
fl ’ 1 t. I A I )—Tl 6:1—2( 1.
J. Olaf av I 1 . Ilvilvuik, St:ivih 21, 195(19(11).
1.5
-
0°(I ).I.u l 0°
i, 2(2)i :to
1). lh irivauivIji:ivi, I I. (1:iscvv, arivl W. Viezee,
1 .29, 1.313
—•—
0° o 18(1°
0.5 to :tn, (1.5 to 40
J. Opt. Sue. Am. 51, 62(1 (1961).
1.34
0.5 to 70
P. 11. Oiese, E. ileltary, K. Ilvillrirli, itral C. I ).
1.5
—-
(J°( 1°)I0°(I()°)
0,2 (0.2) to 159
\ ‘iviiie li Lviii, Al ,Iutval!. I )eiit.. Ak:uI. \Viss.
180°
.
iterlivi, 1(1. .\IalIi. J’li3s. Teilu. 1901, No. 6(1961).
L. l iny—BaI I ian, Mein. lilly. Soc. ( Kk ge)
1 .25
—
0°(2°) 10°( 1(1°)
0. 1(1) 10(1) 20(3) to 50
5cr. (4) 2, Ni ,. 7 (1969).
170°(2°) 18110
11. 11. Giese, Z. A i roplivs. 51, 119 (IlOil),
J. Olaf nail J . 1 ii ,lu i ek, StautI, 21, 495 (1961).
1.27, 1 .33
1 .73, 2.0
1 .08, 0.038
0:18, 1.0
2.25
0.1)5, (1.123
0°(2°) Iv, 18(10
0°( 1°) lo 18(1°
1(1) to 40
1, 2(2) to 30
.
.
0.23
I). Deirir ievulji:ivi, 11.. (3ascui, mii i V. Viozee,
1 .29 to 1.525
0.03, Ui
0° to 18(1°
0.5 to 15 or 25
J. Opt. Soc. A ii i. 51, 620 (1961).
0.043
.
W. J. Pavigoitis avid V. huller, .1 ,ujuler
1 .05 (0.05)
. —
0°(5°) t 180°
0.2(0.2) to 7.0
.S’raucruug /“uilr jonz fur Spherical Pail/des
t 1 .30
(Wayne Uviiver—iiv l’rcss, I)ciroit, 196(1).
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ANGULAR INTENSITY MEASUREMENTS
Figure 5 illustrates the basic optical system used in most light scattering
instruments. }Iodkinson’ 21 ’ discusses several instrument designs and B].ock 787
discusses one design incorporating a laser as a light source.
Fig. 5. Idealized optical system for absolute light—
scattering measurements • 1211
Measurement of particle size by studying the effect of observation angle on
light scattering is possible only in the Mie region. Rayleigh scattering patterns
all have the same angular configuration regardless of particle size. Geometrical
scattering patterns demon rate a correlation with particle size only for very
specialized conditions. 12 Therefore, for practical purposes, there is no
noticeable particle size effect due to geometrical scattering.
Within the Mie range three coon methods are available for correlating
angular intensity variations with particle size. The first method correlates
particle size parameter, , with the number of minima observed in a scattering
pattern such as Figure 2 or 3. The number of minima observed is approximately
equal to for a monodisperse aerosol of sEherical particles as reported by
Johnson and Le Mer 1234 and Barnes, et al.] ISi
The angular location of the first intensity minimum yields a correlation with
particle size as described by Maron and Elder 12 5 and Kerker. 7 ° For a refractive
index near unity and Mie scattering (c > 0.1) the light intensity decreases with
increasing angle of observation, 6, and eventually passes through a minimum at
values of 6 that are dependent on . The following equation describes the
relationship discussed by Maron and Elder 1235 :
d 0
sin (—i) K 1 (2)
where:
61 is the first angular minimum,
d is the particle diameter,
A is the wavelength of incident light, and
K is a conItant that depends on the relative refractive
1 index of particle and medium.
or
Ilydrosol
length I
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Maron, Pierce and Elder 1236 described an extension of the above method
for the first two maxima and minima of the intensity pattern. The same governing
equation is used with subscript “I” corresponding to one of the four cases investi-
gated here:
sin — K 1 (3)
For monodisperse aerosols the diameter obtained is essentially the same for all
maxima and minima observed by Maron, Pierce, and Elder.
For polydisperse aerosols Maron and Elder ’ 235 found a correlation between
the weight average diameter and the intensity extrema. The diameter obtained
at the different maxima and minima is essentially the same as the weight average
diameter obtained from electron microscopy for incident light of low wavelength
(A = 2711 ). The size range covered was 0.18 urn to 7.7 urn.
The third major method of studying the an u1ar dependence of scattered light
intensity is the Sloan—Arrington treatment. 123 ’ 4238 These authors plotted log
vs. log C and observed that a monodisperse aerosol yielded a single maximum on
such a graph while polydisperse aerosols produced as many maxima as there were
dominant particle sizes, as illustrated in Figure 6.
The presence of steep slopes indicates a uniform particle size, while broader
peaks indicate the existence of a wider size distribution. A constant intensity
region in which the scattering curve has a constant slope of —2 indicates that no
particles are present in that size range. The vertical height of a peak is related
to the fraction of particles of this size range.
Livesey and Billmeyer 38 discuss the Sloan—Arrington plots extensively and
conclude that although the method has not yet been fully established on a theoretical
basis and is less accurate than results obtained from other absolute methods, it is
most useful because reasonable size approximations can be obtained readily with a
minimum of computation for both mnonodisperse and polydisperse aerosols. The method
is quite versatile because there are no particle shape and concentration limits.
En addition, the Sloan—Arrington method is applicable over a wide range of particle
sizes and refractive indicies.
APPLICATION TO STACKS
Current light scattering equipment appears to be unsuited for stack monitoring
to determine particle size distribution or mass concentration for three reasons.
First, the measurement procedure is essentially manual and does not lend itself to
the type of automatic operation required for stack monitoring. Second, the data
reduction procedure can be complicated and It h s not been demonstrated that
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results of adequate accuracy can be obtained on stack aerosols. Finally, available
equipment has been developed primarily for research use in the laboratory. Con’
siderably more theoretical and experimental work needs to be done before the
problems and the advantages of this method for stack monitoring can be more
thoroughly assessed. Therefore, angular light scattering is not recommended as
an immediate candidate for stack monitoring.
IN
Fig. 6. Sloan plot of the scattered light intensity from spheres ( ),
discs (—-—), and rods (is. ) according to Livesey and Billmeyer, 38
ANSLAS 100 SO 20 tO S
0.1 O2O 5 I
NADIUI
2 1.0 0.5 0.2
B 20
DISC
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REFERENCES
1232 Barnes, M. D., Kenyon, A. S., Zaiser, E. M., and LaMer, V. K.,
“Monodispersed Sulfur Sole. IV. Comparison of the Particle Radius
Determined by Transmittance and by the Angular Positions of Higher
Order Tyndall Spectra from the Mie Theory”, Journal of Colloid Sciences ,
V. 2, p. 349—359 (1947).
787 Block, A. M., “Scattering of Laser Light by Uniform Spherical Particles”,
Rutgers State Univ., New Brunswick, N. J., Univ. Microfilms order no.
68—4527 (1967).
1233 Bol, J,, Gebhart, J., Roth, C., and Wurzbacher, G., Battelle Institut,
Frankfurt (1969).
702 Call, R. W., Palmer, E. P., and Grow, R. W., “Measurement of Atmospheric
Aerosols by Polarized Laser Light Scattering”, Utah University, Salt Lake
City, Utah, Clearinghouse No. PB 175 688 (Jun 1967).
1211 Hodkinson, J. R., “The Optical Measurement of AerosoiB”, in Aerosol
Science , C. N. Davies (Ed.) Chap. 10 Academic Press (1966).
85 Hodkinson, J. R., “Particle Sizing by Means of the Forward Scattering
Lobe”, Applied Optics , V. 5, no. 5, p. 839—844 (May 1966).
1234 Johnson, I., and LaMer, V. K. “Particle Size of Colloidal Sulfur by Light
Scattering”, Journal of the American Chemical Society , V. 69, p. 1184—
1192 (1947).
70 Kerker, M., Farone, W. A., Smith, L. B., and Matijevic, E., “Determination
of Particle Size by the Minima and Maxima in the Angular Dependence of
the Scattered Light, Range of Validity of the Method”, Journal of Co1lo d
Science , V. 19, p. 193—200 (Mar 1964).
1215 Kerker, M.,The Scattering of Light and Other Electromagnetic Radiation ,
Academic Press (1969).
842 Kratohvil, J. P., and Smart, C., “Calibration of Light—Scattering
Instruments, III. Absolute Angular Intensity Measurements of Mie
Scatters”, Journal of Colloid Science , V. 20, no. 8, p. 875—892 (1965).
38 Livesey, P. J., and Billmeyer, F. W., Jr., “Particle—Size Determination
by Low—Angle Light Scattering: New Instrumentation and a Rapid Method
of Interpreting Data”, Journal of Colloid and Interface Science , V. 30,
no. 4, p. 447—472 (Aug 1969).
1235 Maron, S. H., and Elder, M. E., “Determination of Latex Particle Size by
Light Scattering, I. Minimum Intensity Method”, Journal of Colloid Science ,
V. 18, p. 107—118 (1963).
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1236 Maron, S. H., Pierce, P. W., and Elder, 1 1. E., “Determination of Latex
Particle Size by Light Scattering, III. Minima and Maxima in Angular
Dependence of Intensity”, Journal of Colloid Science , V. 18, p. 391—399
(1963).
805 McCormick, M. P., and Lawrence, J. D., Jr., “Tables of Mie Scattering
Functions for Particles with Refra tive Index 1.5”, Langley Research
Center, Langley Station, Hampton, Va., Clearinghouse No. N 69—20905,
NASA TN D—5110 (Mar 1969).
147 Mis, “Beitrage zur Optik truber medien, spezie].1 kolloidaler Metallosungen”
Annalen der Physik , V. 25, no. 3, p. 377—445 (1908).
474 Shahrokhi, F., and Wolf, P., “Mie—Scattering Function”, University of
Tennessee, Tullahoma, Clearinghouse No. N 68—21227 (Apr 1968).
909 Sinclair, D., and LaMer, V. K., “Light Scattering as a Measure of Particle
Size in Aerosols”, Chemical Reviews , V. 44, p. 245—267 (1949).
1237 Sloan, C. K., “Angular Dependence Light Scattering, Characterization of
Disperse Systems Containing Particles 0.1 to 100 Microns in Radius”,
Paper presented at 125th Meeting of American Chemical Society, St. Louis,
(Mar 24 — Apr 1 1954).
1238 Sloan, C. K., “Angular Dependence of Light—Scattering Studies of the
Aging of Precipitators”, Journal of Physical Chemistry , V.- 59, p. 834—840
(1955).
1201 Vande Huist, H. C, Light Scattering by Small Particles , Wiley (1957).
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SOILING POTENTIAL
by: N. Barsic, B.Y.H. Liu and K. T. Whitby
INTRODUCTION
Interest in the effects of sub—micron, nonsettling particles in the
atmosphere has led to the development of instruments that yield “soiling index”
or “soiling potential” data. Increased industrial activity and automotive
traffic result in airborne particles from incomplete combustion. These small
particles are not removed due to gravity forces and therefore remain in the
atmosphere until they settle on buildings and ,create a dirt nuisance. In addition,
the physiological effects of small particles are more harmful since they can reach
the lungs and remain there rather than being removed by the upper respiratory
tract as are the larger (greater than 5 pm) particles. Effective means of abating
the small particle problem requires a method o detecting their presence.
Most instruments which measure the soiling index basically consist of an air
flow system and a filter. Air passes through the filter for a specified length of
time and the soiled filter is then viewed optically to determine the amount of
material that was deposited. This technique has been adopted by the AISI (American
Iron and Steel Institute) as a means of determiniág pollution levels for the steel
industry. 519
PRINCIPLES OF OPERATION
The most co on instruments utilize a strip of filter paper that is indexed
automatically and evaluated optically at a later time. Figure 1 is a schematic
of the sampler. 396 The original development of this method was pioneered by
Hemeon. 83 ’ The established unit of measurement, called the Coh (coefficient of haze)
Unit, is defined as that quantity of light—scattering solids producing an optical
density of 0.01 when measured by light transmission. Optical density is log 10 (I /1),
where I is the light intensity for lOOZ reflectance or transmittance and I
is the observed reflectance or transmittance.
Axi alternate unit system, Ruda (reflectance unit of dirt), specifies the stain
potential caused by light reflection rather than transmission. This approach
emphasizes the soiling of light colored surfaces which is noticeable due to light
reflectance. The Rud value is that quantity of light—scattering solids producing
an optical density of 0.0]. when measured by light reflection.
Both Rud and Coh values are further standardized to a unit volume of air
filtered through a unit area of filter. The soiling index of the atmosphere is
then defined as Rude or Cobs per 1000 linear feet of air or 1000 cubic feet of air
flowing through one square foot of filter area.
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Dilution air
Fig. 1. Schematic diagram of soiling potential
Sampler (Model #4).396
Tests conducted by Gruber and Schumann 396 utilized the instrument in
Figure 1 with a reflectance meter to evaluate the soiled spots. During these
tests a typical value of soiling potential’ per cubic foot of stack gases was
1.10 Rude—f t 2 /ft 3 . This is equivalent to 134’Rud—ft 2 /lb coal.
The soiling potential correlation with Ringelmann number appears in Figure 2
where both parameters are reported as a function of time. This figure demonstrates
that the soiling potential method is useful as a means of determining smoke opacity
in addition to its original intended use as an a tinospheric dirt measurement method.
Although the soiling index method yields good quantitative results, the
method in its present form is not capable of determining size or mass distribution
data. The method is strictly empirical and ia intended to measure the overall
visual effect of atmospheric pollutants and n t specific characteristics of an
aerosol such as size or mass distribution.
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‘ 10:00 10:30 11:00 11:30 12:00 12:30 1:00 1:30 2:00 2:30
6
= 5,
U
I
10:00 10:30 11:00 11:30 12:00 12:30 1:00 1:30 2:00 2:30
Fig. 2. Soiling potential vs. recorded and observed smoke. 396
Plant #9 — 85,000 lb boiler with spreader stoker.
APPLICATION TO STACKS
The possiblity of obtaining either mass or size distribution data from the
soiling potential recorder is remote since the method was not intended for this
purpose. A correlation between Cohs or Ruds and aerosol mass concentration may
exist. But the relationship is undoubtedly dependent upon the size distribution
of the particles and such particle properties as density and refractive index.
Therefore the method is not considered suitable for either mass concentration or
size distribution measurement in stacks.
RZFERENCES
519 Gruber, C. W., and Alpaugh, E. L., “The Automatic Filter Paper Sampler
in an Air Pollution Measurement Program”, APCA Annual Meetin Paper
No. 54—5, Chattanooga, Tenn., May 3—6, 1954.
396 Gruber, C. W., and Schumann, C. E., “Soiling Potential — A New Method
for Measuring Smoke Emission”, APCA Journal , V. 16, p. 272—275 (1966).
831 Hemeon, W. C. L., “Instruments for Air Pollution Measurement”,
Meteorological Monographs , V. 1, no. 4, p. 20—23 (Nov 1951).
305 Schumann, C. E., and Gruber, C. W., “A Recommended Method for Soiling
Index Surveys by Automatic Filter Paper Sampler”, Paper 60—37 presented
at 53rd Annual Meeting of Air Pollution Control Assoc., Cincinnati, Ohio
(May 22—26, 1960).
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OPTICAL COUNTERS AND PHOTOMETERS
by: B.Y.H. Liu and K.T. Whitby
INTRODUCTION
Optical counters and photometers are widely used as experimental tools
in the study of aerosols. Because of their high sensitivity and their
commercial availability they are now used routinely in such applications as
the monitoring of clean rooms, testing of filters, air pollution studies,
as well as numerous other applications involving aerosols in the laboratory
and the atmosphere.
For the purpose of the present discussion optical counters refer to those
instruments that measure the individual aerosol particles by light scattering
or extinction. As such they are useful for size distribution and concentration
measurements. They are also referred to as single particle optical counters,
aerosol counters, particle counters or dust counters. In the context of the
present discussion, photometers refer to those instruments that perform their
measurement on a cloud of aerosol particles by light scattering. As such they
are useful primarily for concentration measurements. These instruments are
also referred to variously under such names as nephelometers, light scattering
photometers or simply as aerosol photometers.
BASIC PRINCIPLE
The principle of the optical counter can be seen by referring to Figure 1
which shows the optical system of a typical counter available commercially
(Royco Model PC 200). In the counter, aerosol particles are drawn in by suction
through a small tube. As each particle passes through the small illuminated
viewing volume, which is on the order of 1 mm 3 in size, it scatters a pulse of
light which is detected by the photomultiplier tube. The output of the photo—
multiplier tube, in the form of a voltage pulse, provides an electrical signal
whose amplitude is a measure of the particle size. By measuring the amplitudes
of the individual pulses by means of a pulse—heightanalyzer, the size distribution
of the aerosol can be determined.
The history of the aerosol counter dates back to World War II. As a result
of the pioneering studies of Gucker and his co—workers.l 45 ,l 46 ,ll9 2 ,1l93,1194
and similar later studies by O’Konski and associates, 1180 ’ 559 ’ 1253
and Fisher, et al, 288 numerous counters have been developed and are now
commercially available. Table 1 is a List of the manufacturers of aerosol
counters known to the authors. The counters manufactured by the Royco Instrument
Co. have been described by Zinky, 370 , and Ogle, 580 ; those manufactured by the
Bausch & Lomb Instrument Co. are described by Randall & Keller, 12 l 0 Martens and
Keller, 756 Martens and Fuss, 670 and Martens. 578 The instrument manufactured by
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Fig. 1. Optical system of a typical aerosol
counter available commercially.
the Phoenix Precision Instrument Co. is described by Sinclair. 615 In addition
several special purpose counters have been built and described. These include
the special battery—operated portable counter described by Thomas, et al,]- 2 0
and Moroz, et al,l1 28 the high volume (10 to 50 cfm) counter described by
Whitfield and MashburnA 949 and by Neitzel, 824 and the counters described by
Nelson, 276 and Munnna. 5 8 A counter built in Russia has been described by
Kiktenko, et al. 1066
Table 1. List of aerosol counter manufacturers.
1. Bausch & Lomb, 635 St. Paul St., Rochester, New York 14602
2. Climet Instruments, Inc., 1240 Birchwood Drive, Sunnyvale, Calif. 94086
3. Dynac Corp., Thomson’s Point, Portland, Maine 04120
4. Envirco, P.O. Box 6098, Albuquerque, New Mexico 87107
5. High Accuracy Products Corp., 141 Spring St., Claremont, Calif. 91711
6. Phoenix Precision Instrument Co., 3803—05 North Fifth St., Philadelphia, Pa.19140
7. Royco Instruments, Inc., 141 Jefferson Drive, Menlo Park, Calif. 94025
Although most counters make use of an optical system in which only part of
the scattered light is collected by the photomultiplier tube, a cour ter has been
described by Jacobi, et al, 822 in which practically all the scattered light is
collected through a novel lensless optical system using laser illumination and
light collection by a plastic light pipe. Also, it should be mentioned that the
instrument manufactured by the High Accuracy Products Corp. is based on light
* M*
1
— I
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extinction principles rather than on light scattering principles. Another
instrument of a different principle is described by Knollenberg. 876 ” 082
The instrument makes use of a novel arrangement of an optical array consisting
of many small fiber—optical light detecting elements. The size of the particle
is determined by the size of the “shadow” cast by the particle on these elements.
The number of light detecting elements in the shadow area of the particle provides
a measure of the particle size.
The response characteristics of an aerosol counter are generally dependent
upon its optical design. For counters of a specific optical design, the response
of the counter to particles of a specific size is in turn dependent upon the
refractive index of the particle, the geometrical shape of the particle and the
orientation with which the particle enters the optical viewing volume. The
response characteristics of optical counters have been the subject of experimental
studies by several inveBtigators, including Hosey, et al, 993 Channell and Hanna, 988
Lewis, 731 Whitby and Vomela, 171 and Preining, 32 and they have generally confirmed
the facts stated above. Thus, the effect of the particle refractive index is such
that the size of absorbing carbon black particles can be underestimated by as
much as a factor of five by a counter whose calibration is established in the usual
manner by the use of transparent polystyrene latex spheres. The effect of particle
shape is such that a monodisperse aerosol of irregularly shaped particles will be
detected by the optical counter as a polydisperse aerosol because of the dependence
of light scattering on particle shape and orientation. Also, a high concentration
of small particles below the size range of the instrument could scatter sufficient
light to give a false reading 680 . Thus, the response of an optical counter to a
specific aerosol is somewhat empirical and must be established by calibration.
Despite these limitations aerosol counters are widely used because there is
no comparable instrument operating over the range of the optical counter, approxi-
mately 0.3 to 10 tim, that can detect single individual particles. Thus, the
optical counter is specified in Federal Standard 209 for clean room monitoring
(see also Magi11 711 ) and it has been studied 732 , 73 ’ for possible applications to
the monitoring of the cabin atmosphere in manned space crafts.
143,515,362,813
Several theoretical studies 143 have been made on the design of
optical counters. Hodkinson and Greenfield have concluded that, with proper
design, an optical counter using forward scattered light can have the effect of
the particle refractive index minimized.
In order to avoid loss of particle count due to coincidence, i.e., due to
the presence of more than one particle in the sensor viewing volume, the aerosol
concentration must be sufficiently low. Therefore, aerosol with high number
concentrations must be first diluted by clean air before being fed to the counter.
The theoretical basis for calculating the coincidence loss in particle counters
has been thoroughly explored by Bennert and Hulbig. 625
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The light scattering aerosol photometer differs from the aerosol counter
only in the size of the illuminated viewing volume and in the fact that the
aerosol photometer provides a dc electrical signal instead of a pulsed signal
as in the aerosol counter. Because the viewing volume in the aerosol photometer
is considerably larger than that in the aerosol counter, many particles are
present simultaneously in the viewing volume. Thus, the dc ou tput signal of the
photometer provides a measure of the light scattered collectively by all the
particles in the viewing volume. As such, the instrument is useful only for
concentration measurements. A schematic diagram of the commercially available
Sinclair—Phoenix instrument is shown in Figure 2.
locohon
o H — Scaled Light Source Housing
°B —Lamp
o D — Feathered Edge Diaphragm
• L — Condenser Lens
o L’ — Condcnser Lens
o D’ — Diaphragm Stop Produces converging cone of darkness
o iT — Filtered Air Inlets
o FC — Optical System protector filter and carrier
• E — Removable eyepiece which replaces photomuhipliez tube
for visual observation of smoke particles, also used for
alignment
• CFC — Mass collector filter carrier
• A — Stray Light Limiting Diaphragm
• PA — Filament Alignment Window
Fig. 2. Schematic diagram of the Sinclair—Phoenix
aerosol photometer.
ROOM AIR INTAKE
FILTERS -
CLEAN COOLING AIR
PASSED BY FILTER PC
AND CFC GOING TO
H FA
RESTRICTION TUBE
325 MESh SCREEN
p.
TUBING
CFC
_______ MOLECULAR FILTER COLLECTING DUST LOAD FOR
MICROSCOPIC EXAMINATION (IF DESIRED)
Location
• C — Converging Cone of Light
• ST — Smoke Inlet Tube
• ST — Smoke and Filtered Air Outlet Tubs
• H’ — Diverging Cone of Darkness
• S — Stray Light Diaphragm
• 0 — Optical Calibration Filters
• L’ — Scattered Light Collecting Lens
• I’ —Light Trap Tube
• W — Photomultiplier Tube Diaphragm
• P — Photomultiplier Tube
• P ’ — Shielded Photomulciplier Tube Houssog
• S ’ — Sealed Optical Housing
• PS — Cable t o Logarithmic Amplifier
• FAO — Lamp Coolini Air Inlet
• RT — Rotatable Ligli Trap 71k. hangs
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The basic design requirements of the aerosol photometer suitable for
laboratory use are described in considerable detail in the Naval Research
Laboratory report by Knudson and White. 84 ° Further studies of the aerosol
photometer have been described by Mueller and Givens 374 and by Doyle and
Wiederhorn 1 285 The response charactcristics of the aerosol photometer as
a function of particle size for a Vho oineter of Soviet design have been
described by Kogen and Pankratova.LO 8 The design and the application of a
total scattering aerosol photometer, called the integrating nephelometer,
to the measurement of the meteorological range and atmospheric aerosol con-
centration have been described in a series of papers by Charison and his co-
workers. 928 , 69 ,919
The response characteristics of the aerosol photometer are complicated due
to their dependence on the size, the shape, and the optical design of the
instruments. Generally, for a specific instrument the dc signal provides a
measure of the aerosol mass concentration only when the characteristics of the
particles, including the refractive index, the density and the size distribution,
remain unchanged. For application to stacks where all of these parameters can
and do vary, the instrument cannot provide a measure of aerosol mass.
Although the aerosol photometer can also be used to determine the size
distribution by combining its operation with an impactor as Thonipson 124 has done,
the data is difficult to convert to either the number distribution or the mass
distribution because of the c ompllcated nature of the relation between particle
size and light scattering.
APPLICATION TO MASS CONCENTRATION MEASUREMENT IN STACKS
Single particle optical counters are unsuitable for continuous mass con-
centration measurement In stacks for several reasons. The Instruments measure
the size distribution of particles, making the readout quite difficult to handle.
Most practical instruments have a rather narrow size range. An instrument designed
for large particles has a large sensing zone which makes it insensitive to small
particles. In order to relate the measured number concentration to particle mass
concentration, the specific density of the particles must be assumed. Specific
particle density changes drastically in effluents. The size classification
depends strongly on the refractive index of the particles. Nearly all present
commercial instruments are designed for clean room atmospheres. Design for
aerosols as concentrated as stack effluents would require very high dilution (e.g.,
1,000 — 10,000 X dilution) with clean air to prevent coincidence counting errors.
Aerosol photometers are also unsuitable for continuous mass concentration
measurement in stacks. They share many of the problems of transmissometers for
mass concentration measurements. The sensitivity of photometers is higher than
transmissoineters, a feature not necessary for stack effluents. The greatest
disadvantage of a photometer compared to a transmissometer is that a photometer
requires an extracted sample, Introducing another large measurement uncertainty.
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SUMMARY AND CONCLUSIONS
Optical counters and aerosol photometers have been used as experimental
tools in numerous aerosol studies. The optical counter measures the size of
a particle by detecting the amount of light scattered by the particle in
passing through the small illuminated sensing volume whereds the aerosol photo-
meter measures the light scattered collectively by all the particles present
in the illuminated viewing volume. The optical counter can-be used for size
distribution measurements whereas the aerosol photometer is suitable primarily
for concentration measurements. Optical counters are unsuitable for mass con-
centration measurement in stacks because of their sensitivity to the refractive
index of the particles, the limit in the aerosol concentration, their inability
to sense particle density, their inconvenient data readout and several other
reasons. Aeroso-l photometers are unlikely candidates for stack mass monitoring
because they have no specific advantage, but do have a large disadvantage,
over the transmissometer.
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988 Channell, J. K., and Hanna, R. J., “Experience with Light Scattering
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919 Charison, R. J., Ahiquist, N. C., Horvath, H., “On the Generality of
Correlation of Atmospheric Aerosol Mass Concentration and Light
Scatter”, Atmospheric Environment , V. 2, p. 455—464 (1968).
69 Charison, R. J., “Atmospheric Visibility Related to Aerosol Mass
Concentration, A Review”, Environmental Science & Technology , V. 3,
no. 10; p. 913—918 (Oct 1969).
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928 Charison, R. J., Ahiquist, N. C., Selvidge, H., and MacCready, P. B.Jr.,
“Monitoring of Atmospheric Aerosol Parameters with the Integrating
Nepheloineter”, APCA Journal , V. 19, no. 12, P. 937—942 (Dec 1969).
285 Doyle, A. W., and Weiderhorn, N. M., “The Use of a Light—Scattering
Photometer in Air Pollution Studies”, paper presented at 3rd Annual
Meeting, New England Section, Air Pollution Control Assoc., May 14,1959.
813 Dunn, J. F., Jr., Foster, A. R., Lautman, D. A., Nardone, L. J., Swanson,
J. L., and Zelinski, J. J., “On the Design of an Instrument to Use
Forward—Scattered Light to Detect Small Particles”, NortheasteniUniv,,
Boston, Mass., Clearinghouse No. AD 693 557 (Aug 1969).
225 Duwel, L., “Latest State of Development of Control Instruments for the
Continuous Monitoring of Dust Emissions”, Staub—Reinhalt der Luft
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288 Fisher, M. A., Katz, S., Lieberman, A., and Alexander, N. E., “The
Aerosoloscope; An Instrument for the Automatic Counting and Sizing.
of Aerosol Particles”, Proc. 3rd National Air Pollution Symposium ,
Pasadena, Calif., p. 112—119 (Apr 18—20 1955).
145 Gucker, F. T., Jr., O’Konski, C. T., Pickard, H. B., and Pitts, J. N.,Jr.,
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146 Gucker, F. T., Jr., and O’Konski, C. T., “An Improved Photoelectric
Counter for Colloidal Particles, Suitable for Size—Distribution Studies”,
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1192 Gucker, F. T., and Rose, D. G., J. 4ppl. Phys. Suppl. 3 , (1954).
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(1955).
1194 Gucker, F. T., Proc. md. Instr. Conf. , p. 284 (1956).
515 Gucker, F. T., and Tuma, J., “Influence of the Collecting Lens Aperature
on the Light—Scattering Diagrams from Single Aerosol Particles”, Con-
tribution No. 0000 from the Chemical Laboratory of Indiana University,
Bloomington, Indiana, 47401.
1110 Gucker, F. T., “Instrumental Methods of Measuring Mass Concentration
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752 Harris, F. S., Jr., Sherman, G. C., and Morse, F. L., “Experimental
Comparison of Scattering of Coherent and Incoherent Light”, IEEE Trans.
onAntennas and Propagation , V. AP—15, no. 1, p. 141—147 (Jan. 1967).
143 Hodkinson, J. R., and Greenfield, J. R., “Response Calculations for
Light—Scattering Aerosol Counters and Photometers”, Applied Optics ,
V. 4, p. 1463—1474 (1965).
993 Hosey, A. P., Jones, H. H., and Ayer, H. E., “Evaluation of an Aerosol
Photometer for Dust Counting and Sizing”, American Industrial Hygiene
Assoc. Journal , V. 21, p. 491—501 (Dec 1960).
822 Jacobi, W,, Eichlers, J., and Stolterfoht, N., “Particle Size Spectrometry
of Aerosols Through Light Scattering in a Laser Beam”, Sandia Lab.,
Albuquerque, N. M., Clearinghouse No. PB 187 7051 (Nov 1969).
307 Kay, K., “Analytical Methods Used in Air Pollution Study”, Industrial
and Engineering Chemistry , V. 44, p. 1383—1388 (1952).
1066 Kiktenko, V. S., Safronov, Y.P., Kudryautsev, S. I., FedorOv, B. F.,
Pushchin, N. I., and Fedorovich, A. A., “Photoelectric Count of the
Number of Aerosol Particles of Organic and Inorganic Origin”, Engi.
Trans. from: Gi iyenai Sanitariya 1 Moscow , V. 26, no. 2, p. 47—53,
Feb. 1961, Clearingh6use No. JPRS—8334 (May 1961).
876 Knollenberg, R. C., “An Optical—Electrical Particle Size Discriminator”,
University Corp. for Atmospheric Research, Clearinghouse No. PB 179 645
(Apr 1968).
1082 Knollenberg, R. G., “The Optical Array: An Alternative to Scattering or
Extinction for Airborne Particle Size Determination”, Journal of Applied
Meteorology , V. 9, no. 1, p. 86—103 (Feb 1970).
840 Knudson, H. W., and White, L., “Development of Smoke Penetration Meters”,
U. S. Naval Research Laboratory, Washington, D. C., NRL Report P—2642
(1945).
1083 Kogan, Y. I., and Pankratova, M. E., “Dependence of Light Scattering on
Particle Size in Aerosol Nephelometers with Viewing Angles of 90 and
135 Beg.”, Colloid Journal, USSR (Engl. Trans.) , V. 31, no. 4, p. 496—500
(Jul—Aug 1969).
731 -Lewis, T. W., “Evaluation of an Automatic Aerosol Particle Counter for
Measuring-the Airborne Contamination Level in a Controlled Environment”,
George C. Marshall Space Flight Center, Huntsville, Ala., Clearinghouse
No. 66 29075 (Mar 1966).
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71.1 Magill, P. L., “Instrument Measurement of Contamination in Clean Rooms
for Proposed Federal Standard No. 209”, Proc. National Analytical Inst.
Symp., 10th. San Francisco, Calif. , p. 287—297 (1964).
578 Martens, A. E., “Errors in Measurement and Counting of Particles Using
Light Scattering”, AP A Journal , V. 18, p. 661—663 (Oct 1968).
670 Martens, A. E., and Fuss, K. H., “An Optical Counter for Dust Particles”,
Staub—Reinhalt der Luft , V. 28, no. 6, p. 14—18 (Jun 1968).
756 Martens, A. B., and Keller, J. D., “An Instrument for Sizing and Counting
Airborne Particles”, American Industrial Hygiene Assoc. Journal , V. 29,
P. 257—267 (May — Jun 1968).
1128 Moroz, W. J., Withatandley, V. D., and Anderson, G. W., “A Portable
Counter and Size Analyzer for Airborne Dust”, Review of Scientific
Instruments , V. 41, no. 7, p. 978—983 (Jul 1970).
374 Mueller, P. K., and Givens, R. G., “Dynamic Calibration and Data
Interpretation of a Light—Scattering Instrument”, APCA Journal ,
V. 11, no. 12, p. 576—580, 584 (Dec 1961).
598 Mtnmn , V. K., Thomas, A. L.,Jr., and Collins, R. H., III, “A Particle
Size Analyzer for Aerosols”, Army Biological Labs., Frederick, Md.,
Clearinghouse No. AD 636 858 (1962).
969 Nader, J. S., Ortman, G. C., and Massey, M. T., “Light—Scatter
Instrumentation for Measurement of Atmospheric Particulates”, American
Industrial Hygiene Ass’n. Journal , V. 22, no. 1, p. 42—48 (Feb 1961).
824 Neitzel, V. E., “A High—Volume, Real—Time Aerosol Monitor”, Sandia Lab.,
Albuquerque, N. M., Clearinghouse No. SC—DR—69—56 (Jun 1969).
276 Nelson, M. B., “Fabrication of Particle Counters for Clean Rooms”, lIT
Research Institute, Chicago, Ill., NASA Tech. Brief 67—10076. (Jul 1966).
580 Ogle, H. M., “Particle Counting Techniques Available for Aerosol Research”,
APCA Journal , V. 18, p. 657—699 (Oct 1968).
1180 O’Konski, C. T., and Doyle, J., “Light—Scattering Studies in Aerosols
With a New Counter—Photometer”, Analytical Chemistry , V. 27, p. 694—701
(May 1965).
559 O’Konski, C. T., Bitron, M. D., and Higuchi, W. I., “Light—Scattering
Instrumentation for Particle Size Distribution Measurements”, American
Society of Testing Materials , Spec. Tech. Pub. No. 250, p. 2—28 (1959).
1253 O’Konski, C. T., Bitron, M. D., and Higuchi, W. I., ASTM Spec. Tech. Pubi.,
No. 234 , (1958).
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32 Preining, 0., “The Cross Sensitivities of the Royco Aerosol Photometer
PC200”, Staub—Reinhalt der Luft (Engl, Trans.) , no. 1, p. 29—32 (Jan 1968).
1210 Randall, L. M., and Keller, J. D., “Electro—Optical Aerosol Countet Inst.,”,
AIHA Conf., Houston, Texas (1955).
340 Setzer, D. E., “Comparison of Measured and Predicted Aerosol Scattering
Functions”, Applied Optics , V. 8, no. 3, p. 905—911 (May 1969).
615 Sinclair, D., “A New Photometer for Aerosol Particle Size Analysis”,
APCA Journal , V. 17, no. 2, p. 105—108 (1967).
287 Anon, “Reference Manual for the Southern Research Institute Particle
Size Analyzer”, Southern Research Institute, Birmingham, Alabama
(Oct 1959).
120 Thomas, A. L., Jr., Bird, A. N., Jr., Collins, R. H., III, and Rice, P.C.,
“A Portable Photometer and Particle Size Analyzer”, ISA Journal , V. 8,
p. 52—56 (Jul 1961).
124 Thompson, J. K., “Determination of Aerosol Size Distributions by Jet
Impactor—Light Scattering Technique”, Analytical Chemistry , V. 29,
p. 1847—50 (1957).
362 Welch, E. C., “Design of an Optical System for Counting and Sizing
Airborne Particles”, ISA — Nat. Symposium on Instrumental Methods
of Analysis, 8th — Proc. , j. 173—178 (1962).
171 Whitby, K. T., and Vomela, R. A., “Response of Single Particle Optical
Counters to Nonideal Particles”, Environmental Science & Technology ,
V. 1, no. 10, p. 801—814 (1967).
680 Whitby, K. T., and Liu, B. Y. H., “Generation of Countable Pulses by
High Concentrations of Subcountable Sized Particles in the Sensing
Volume of Optical Counters”, Journal of Colloid & Interf ace Science ,
V. 25, no. 4, p. 537—546 (1967).
949 Whitfield, W. J., and Mashburn, J. C., “Development of an Increased
Sampling Rate Monitoring System”, Sandia Lab., Albuquerque, N. M.,
Clearinghouse No. SC—RR—66—585 (Oct 1966).
370 Zinky, W. R, “A New Tool for Air Pollution Control, the Aerosol Particle
Counter”, APCA Journal , V. 12, no. 12, p. 578—583 (Dec 1962).
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LIDAR
by: N. Barsic, B.Y.}L. Liu, and K.T. Whitby
I2 TR0DUCTI0N
Lidar is a relatively new technique that detects particles of dust and other
materials in the atmosphere. This method of particle detection is sensitive
enough to pinpoint atmospheric debris in what appears to be clean air. Lidar has
already provided valuable information in many types of atmospheric investigations.
“LIDAR” is the optical analogue of radar, and the word is formed in the same
way: from light detection and ranging. This technique uses light energy from
lasers rat1 r thin radio w vesThs with radar.
BASIC OPERATIONAL PRINCIPLES
A. Objective
The goal of atmospheric studies using lidar is to determine the presence of
particulate contaminants, establish their concentration, and determine the cloud
boundaries. Successful results are reported In numerous Instances. 28 O, 221 569 , 514
Mostapplications involve the study of cloud or pollution formation at distances
of 100 m to 100 km from the lidar unit.
The basic concept of lidar operatIon, like radar, involves sending energy
pulses toward an object, and detecting the reflected energy. Energy generated by
large, Q—switched lasers used in atmospheric studies is concentrated in very short
duration (30 x iO- sec), monochromatic, high power pulses (45 megawatts). These
energy pulse beams are directed by either reflecting or refracting telescopes.
Energy returned by the atmosphere is collected by a similar telescope and sensed
by a photomultiplier.
1 ’. e’
V
Li ... W.d. .. ._ ,_ 43
ten’,,
Pi.te.øp R.H.dIM b”. ’
Fig. 1. A typical lidar system
- — ., ,. _,,.1*,
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B. Theory of Operation
A typical lidar system is shown in Figure 1., from Collie (1969). The
primary equipment consists of a laser, sending telescope, receiving telescope,
and signal detector. Most lidar units are constructed in this manner 286 , 8 O 2 , 22 1
The laser is generally a 0.6943 i.iui wavelength ruby laser or a 1.06 .im wave-
length neodymium—glass laser. Other laser types have been used successfully,
however, more data are available from experiments conducted with these two types.
A lidar signal is affected by particles that are larger than one—tenth of the
laser light wavelength. 66 Consequently, a ruby lidar is sensitive to particles
greater than 0.07 urn diameter, the neodymium lidar to particles greater than
0.1 urn diameter. Since most of the mass of atmospheric aerosols is associated
with particles larger than these two limits, either laser appears suitable.
However, if small particles must be sensed, the advantage of the ruby lidar is
obvious. The neodymium laser is operationally safer because it operates at
wavelengths above the visible range.
The sending telescope and receiving telescope are usually of the same
construction. A cassegrain telescope is most commonly used. In addition to
telescopes, a simple optical system is necessary to direct the laser beam to
the telescope, and to direct the return signal to the photomultiplier. Part of
the return optical system must include a narrow band filter to eliminate solar
noise.
The governing equation for lidar detection of atmospheric targets appears
below:
(c ‘ 5/2) B A/r 2 exp [ —2 f o(r)dr]
where
is received power,
is transmitted power,
c is the velocity of light,
‘5 is pulse duration,
r is range,
B is the volume backscattering coefficient of the atmosphere
at range r (dimensions: area/unit volume/steradian),
A is the effective receiver aperture, and
a Is the volume attenuation coefficient.
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All values in the above equation are either known or can be calculated
except the volume backscattering coefficient, 8, and the volume attenuation
coefficient, a, Both of these coefficients are functions of the number con-
centration, size distribution, and optical properties of the atmospheric
aerosol. If the size distribution and optical properties are measured or
assumed, then the aerosol mass concentration can be evaluated from a lidar
return. The greatest difficulty concerning most lidar users is to determine
the size distribution and optical properties of an aerosol. The best method
is to use some other independent means of obtaining this information. However,
the cost of the project then grows rapidly. Many investigators have estimated
the size distribution and optical properties from the literature,but this
approach obviously results in some error.
Once the optical properties and size distribution are established, a
relationship between the backscattering coefficient, $, and the attenuation
coefficient, a, is necessary. Several methods of determining the functional
relationship between a and B are presented in the literature.2 21 The single
lidar equation can then be solved for the one remaining unknown, 8.
The method of calculating mass concentration as function of the scattering
coefficient, 8, is discussed by Barrett and Ben—Dov) ’ 9 In addition to mass
concentration, the author also defines turbidity and visibility from lidar returns.
A lidar observation consists of evaluating a received signal, r’ as a function
of range and direction. The minimum detectable signal is limited by the system
noise, solar energy noise, and detector sensitivity.
The data signal is generally displayed on an oscilloscope or some permanent
record. Typical signals are shown in Figure 2.
zns’t,
Fig. 2. Typical Lidar signal.
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Most current lidar units are designed to monitor pollution level from
distances of 100 in to 100 km. Two investigators, Cooper and Byers, 89 monitored
a laboratory aerosol confined to a 3/8 inch diameter stream. They used a Ne—He
gas laser and measured the backscattered light from salt particles. Concentrations
measured were about ig/m 3 and the mass median diameters ranged from 0.22 jim
to 0.70 .ini. Although current use of lidar (backacattering) concerns monitoring
pollution from a distance of several hundred meters, this investigation demonstrates
the possibility of using lidar for short range operation as well. The limiting
condition for short range operation is a function of the speed of light and minimum
laser pulse duration, which is currently in the nanosecond region. For short
range operation, the lidar unit must be located far enough from the source so that
the light pulse terminates before the initial radiation reaches the target. Current
designs therefore require a minimum lidar—to—target distance of a few hundred
centimeters.
Resolution limits of lidar studies concern both particle size and concentration.
There is probably no reasonable limit to the maximum concentration that can be
detected by lidar. Some of the higher concentrations reports are 50,000 jig/m 3
by Cooper and Byers. 89 The lower resolution limit and the ability to detect
concentration gradients are more difficult to define. Lower resolution limits are
a function of the lidar system ua1ity and are subject to change due to technical
advances. Barrett and Ben—Dov 1 9 reported detecting concentrations as low as
5 jig/rn 3 and variations as small as 2 jig/rn 3 .
The particle size resolution limit is determined by the laser wavelength
since Mie scattering is considered to be the governing mechanism. Particles with
a diameter that is greater than one tenth of the laser light wavelength can be
detected. 66 Since most lidar units operate at 0.6943 jim or 1.06 jim, the lower
limit of particle size resolution should be about 0.07 .im. Experiments conducted
with laboratory aerosols have demonstrated the ability of lidar to detect particles
of 0.22 jim mass median diameter. 89 Studies made on atmospheric aerosols indicate
that particles of smog, haze, and smokes can be detected by lidar. 22 l
In addition, Collis indicates that it may even be possible to detect gaseous
molecules with lidar. An obvious qualification for the ability to detect gaseous
molecules is that the foreign gas be considerably different from the environment
through which the lidar signal travels.
APPLICATION TO STACKS
Lidar can be used as a means of evaluating the mass concentration of an
aerosol, however, some knowledge of the particle size distribution is necessary.
Consequently, either additional effort must be expended to determine the size
distribution, or an estimate of size distribution must be made, and particle
density must be known.
Lidar has been used to measure the concentration of a laboratory aerosol, so
it appears feasible to use this method in a stack. Slight instrument modifications
are necessary, such as using the shortest possible laser pulse rate, and locating
the lidar unit far enough away from the stack to prevent the signal from being
backscattered before the entire forward signal has been sent.
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Although lidar seems to be an excellent means of defining atmospheric
contamination, three factors make lidar unsuitable for “in—stack” monitoring.
The first factor is cost. Virtually all Lidar units are best described as
research devices. Therefore, cost information is not readily available, but
is somewhat higher than the cost of a high quality Q—switched ruby laser. The
second prohibitive feature of lidar is reliability. Although it has been used
successfully, lidar is still a research device, and probably not well—suited
to continuous monitoring on a daily basis. Finally, current lidar units are
designed to study atmospheric contaminants from a distance of at least 100 in.
A limited amount of research has been conducted with close range ].ldar. There-
fore, additional studies are needed before lidar can be considered for stack
monitoring. In any case, lidar cannot measure the particulate mass concentration
directly. It requires additional information about particle size distribution
and density to correct its basic measurement for particle mass.
REFERENCES
149 Barrett, E. V., and Ben—Dov, 0., “Application of the Lidar to Air
Pollution Measurements”, Journal of Applied Meteorology , V. 6,
p. 500—515 (1967).
280 Barrett, E. V., “Lidar Measurements of Particulate Concentration Profile&
Paper 67—128 presented at 60th Annual Meeting of Air Pollution Control
Assoc., Cleveland, Ohio, June 11—16,1967.
569 Collis, R. T. H., “Lidar: A New Atmospheric Probe”, Royal Meteorological
Society, Quarterly Journal , V. 92, p. 220—230 (1966).
221 Collis, R. T. H., “Lidar for Routine Meteorological Observations”,
American Meteorological Society Bul. , V. 50, no. 9, p. 688—694
(Sep 1969).
89 Cooper, B. V., and Byers, B. I., “Laser Light Backscattering from
Laboratory Aerosols”; APCA Journal , V. 20, no. 1, p. 43—47 (Jan 1970).
66 Johnson, W. B., “Lidar Applications in Air Pollution Research and
Control”, APCA Journal , V. 19, no. 3, p. 176—180 (Mar 1969).
802 Landry, M. J.,, “GB—60A Light Detecting and Ranging System (LIDAR)”,
Sandia Laboratories, Albuquerque, N. M., Clearinghouse No. SC—DR—67—1l5
(Dec 1967).
286 Northend, C. A., Honey, B. C., and Evans, V. E., “Laser Radar (Lidar)
for Meteorological Observations”, Review of Scientific Instruments ,
V. 37, no. 4, p. 393—400 (1966).
514 Proctor, T. D., “A Laser Technique for the Measurement of Aerosols”,
Journal of Scientific Instruments (Journal of Physics E.), Ser. 2, V. 1,
no. 6, p. 631-635 (Jun 1968).
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HOLOGRAPHY
by: N. Barsic, B.Y.R. flu, and K.T. Whitby
INTRODUCTION
Holography is an interferometric technique by which three dimensional
information can be recorded on a two—dimensional photograph. This is a step
process consisting of first photographing the interference pattern that exists
when a diffracted or object field (Fresnel or Fraunhofer diffraction pattern
of the object) is allowed to interfere with a reference field or background
wave. Holography has received considerable attention recently due to the
availability of high power, monochromatic lasers as an illumination source.
Holography can be used in several ways to measure small particles. The
first is the photography of individual particles by holography for subsequent
viewing f or size distribution or shape analysis. The second is the photography
of clouds of particles in three—dimensions. Analysis of the intensity of the
reconstructed image can yield some information regarding the particle concentration.
The third is the photography of light scattered by the particles, either individu-
ally or as a cloud. Analysis of the intensity of light scattered by individual
particles can yield size information of particles as small as 1 — 10 pm. Analysis
of the cloud intensity can yield three—dimensional particle concentration informa-
tion.
As a means of measuring the particle size and number distribution, holography
has two advantages. First, this method enables a size distribution analysis to
be made without disturbing the medium under consideration. Secondly, a complete
description of the three dimensional sample volume is possible, rather than sampl-
ing at a particular point in space.
Problems include the cost and complexity of the necessary apparatus, the
difficulty in detecting individual particles as small as 1 — 10 urn across stack—
like dimensions, and the lack of ability to detect individual submicron particles.
Holography is a complex science in an infant state of development. Future
developments will undoubtedly improve present performance and reduce the cost and
complexity of the equipment.
Holography is resently being developed for use in effluent stacks and steam—
plant boilers. 279 ’ 257 Results to date demonstrate that holography can be used
in such environments. Although the method is costly and data reduction is time
consuming, holography offers an excellent method of aerosol classification for
research work where accuracy is necessary and the flow field cannot be disturbed.
Little application is seen for holography as a continuous monitor.
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PRINCIPLES OF OPERATION
A. Objective
Holography can be used as a means of determining the size and number dig-
tribution of an aerosol that passes through a given volume. The reconstructed
holograms can be displayed on a TV monitor and can then be photographed for
observation or data reduction. The Stat Volt Company 1198 also reports the
availability of an electronic scanner that yields a digital output of a number
and size distribution on a screen or printed on paper.
Another more direct method of analyzing holograms for either particle size
or concentration as a function of location within the viewing volume is to use
a photocell, or photomultiplier with a small aperture. The photo detector can be
moved around in the reconstructed viewing volume, scanning the volume for particles
or for changes in light intensity ) ’ 257 Once the light amplitudes are converted to
electrical signals, a variety of data analysis possibilities exist. The result
could be a size—number distribution of the entire volume or any portion of the
volume or a three—dimensional spatial distribution of the light—scattering intensity
of a cloud.
B. Theory of Operation
1. Introduction
Two types of holograms are the Fresnel or near—field and the Fraunhofer or
far—field. The distinction between Fraunhofer and Fresnel holograms lies in the
nature of the diffracted radiation from the object. A Fraunhofer diffraction
pattern is always formed when the point of observation is infinitely distant from
the object, while a Fresnel pattern if formed when the observation point is near
the object. The original holograms were photographic records of interference
patterns between Fresnel diffraction and a reference beam. One unfortunate con-
sequence of a Fresnel hologram is the existence of a conjugate image (located near
the hologram) that degrades the reconstructed image. Fraiunhofer holograms produce
a clearer reconstructed image since the conjugate image is essentially located at
infinity.
A hologram is formed by allowing a laser beam to fall upon a reflecting
surface and an object so that both the reflected and diffracted beams intersect
at a plane in space that can be photographed. One possible arrangement is shown
in ‘Figure 1. Other arrangements of the object, reflecting surface, and photographic
plate are possible.l 28 , 1 ’ 4 S 483 The essential feature is that a Teference beam and
a diffracted beam intersect in a plane where a photograph can be taken.
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plate
r
Fig. 1. Arrangement for producing a hologram
from C. R. Fowles.U 95
2. Hologram Formation
The basic experimental schematic for a Fraunhofer hologram is illustrated
in Figure 2, from Thompson, et al. 1 - 5 °
1.
.4
.-.—:..,
—1- • ••
‘ . hiN(,I’i i
Fig. 2. The basic principle of the disdrometer. Partially
coherent, quasi-monochromatic radiation Illuminates
the sample volume of particles. The hologram of a
particular particle is recorded on film placed In
the x, y plane a distance z from the particle.
Quasi—monchromatic radiation illuminates the sample volume which is located at
a distance Z from the recording plane. The distance Z must satisfy the following
equation in order to produce a Fraunhofer diffraction pattern 682 , 2 3
2 2
- ( + n ) max
(1)
Photographic
—-0-
y
A
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where A is the mean wavelength of illuminating radiation and (C 2 + it 2 ) max is
the maximum cross sectional dimension of the object.
2
4a (2)
for spherical objects of diameter 4 a ( + n )
holography is a two—step imaging process utilizing diffraction techniques
by means of partially coherent radiation. The theoreticaL basis for holography
depends upon the solution of a simultaneous pair of wave equations with appropriate
boundary conditions. The measureable quantity described in this wave equation is
related to the illumination intensity. Details of the wave equation boundar
conditions, and solution are discussed by Be].z 682 and Reynolds and DeVelis. 12
The results of this solution are presented in the following equation which defines
the intensity distribution for a particle of diameter 2r:
2
, kap , kap
2 J ‘Z ‘ 2 2 2 J ‘Z
2ka 1 1. kp ka 11
t(p) L— Z k 2 k
1 1 ( z
1 1
where
a is the particle radius in the diffracting plane,
p is the particle radius coordinate in the observation plane (hologram)
and 0 where corresponds to radius a,
k is 2 n/A,
X is the wavelength of the illuminating radiation,
is the distance from the observation plane to the diffracting
plane (recording distance), and
is the Bessel function of the first kind.
The first term is simply a constant normalized intensity about which the
diffraction pattern is formed. The second term dominates and represents the
interference between the Fourier transform of the particle geometry and the
coherent background. The third term is the intensity distribution in the
Fraunhofer diffraction pattern of an aperture having the same cross sectional
dimensions as the particle. This term is small and is frequently omitted.
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The above description of the important equations and parameters provides
the basis for completing step one (hologram formation) of this two—step imaging
process. Thus far, only a film record (hologram) of a diffraction pattern has
been formed. The second and final step, reconstruction, yields a three—dimensional
image of the particles.
3. Reconstruction
Holographic recording stores information about an object in the form of an
interference pattern which appears to consist of a seemingly meaningless array
of lines. Reconstruction is a procedure that obtains a three—dimensional image
of the object from the interference pattern.
This is accomplished by illuminating the film record obtained during the
hologram formation step with a coherent beam of quasi—monochromatic light as
illustrated in Figure 3.
17
Fig. 3. The basic principle of the readout. The hologram located in
the x, y plane Is Illuminated by partially coherent quasi—
monochromatic light. The hologram of a particular particle
reconstructs a real image of the particle at a distance z.
The distance z, size and shape of the real Image and its
location in the C—n plane are uniquely determined by the hologram.
This ght beam is generally collimated, but can be either converging or diverg-
ing. 1 For a quasi—monochromatic collimated beam of the same wavelength as the
beam forming the diffraction pattern, the reconstructed image is located at a
distance from the hologram. Under these conditions the magnification is unity.
If light of a different wavelength Is used, then the reconstructed image lies
in a different plane, but the magnification Is still unity.
C
- -
—
2
I .N••. t nI
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The intensity distribution of the reconstructed Image is obtained by solving
a pair of wave equations similar to those solved in step one (hologram formation).
Equation 4 describes the solution for a particle of radius a.
2 2
1(r) 1 + D(r) cos 2 X ) + (D(r)1 (4)
2
where
r is the particle radius coordinate in the image plane and
0 < r < a, where a corresponds to p,
P is the particle radius in the hologram plane,
k ia2it/A,
A is the wavelength of illuminating radiation,
Z 2 is the distance from the image plane to the hologram plane
(generally Z 1
D(r) is related to the cross sectional geometry of the particle, and
y is the photographic gRtmnR, Reynolds and DeVelis. 126
This equation indicates that the cross sectional geometry of the original
object is recovered.
4. Discussion
Analysis of a hologram at a single random location within the viewing volume
is not representative of the aerosol. Instead, several locations within a holo-
gram are necessary to completely classify an aerosol. The number of separate
locations that must be analyzed across a viewing volume depends on the aerosol
type, velocity profile, and concentration profile. To obtain a meaningful
statistical analysis of an aerosol, the reconstruction analyzer is usually
located on a movable carriage that allows continuously spaced successive planes
to be in focus on the analyzer. The analyzer can be a camera, a CCTV camera, or a
photodetector.
Th MncVeTcuc n ir
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For each recording distance Z there is a minimum resolvable particle
diameter resulting in a hologram c ntrast that is low enough to recede into
the background noise level of the film, and not be recorded properly. The noise
level is a constant that depends on the film and the quality of the optical
system. This fact is especially important if a long sample volume must be
transversed or if the recording distance must be long due to the nature of the
experiment. The recording distance requirement is summarized in Figure 4 which
illustrates the minimum resolved particle diameter as a function of recording
(far—field) distance.
If the far—field distance can be controlled in favor of small particles, the
resolution limit f or present techniques is about 2 m. 1197 This limitation is
set by an approximation made in the derivation of Eq. 3, namely that the dimensions
of the diffracting object be larger than the radiation wavelength. For a ruby
laser this limit must be much larger than 0.6943 urn.
Ultraviolet pulsed gas lasers are available that produce sufficient power
to be used as a light source. With a nitrogen plasma laser and a suitable
filtering system, a strong line at 3371 can be obtained. This reduction in
wavelength allows 1 im diameter particles to be observed, however, the far—field
distance must be increased accordingly (Eq. 2).
Lensless holography, i.e., with no lens between the viewing volume and the
photographic film plate, has a magnification of 1 and has a maximum resolution
of 15 — 25 uim, depending on the quality of the film. The depth of field of
lensless holography can be 30 ft. or more, enough to cross most stacks. However,
the resolution as seen in Figure 4 deteriorates at such distances.
; - 4.
U
U
c
0
0
0
U_ i
I -- I
3 5 10 15 20
Particle drnmcter. mkron ( j)
Fig. 4. Comparison of experimental and theoretical results
for particle size resolution (using the contrast
criterion) versus far—field recording distance. All
particles fa1lin below the curve are withit the
recording range lSO ,l 32
-. I II( I(’ i •iI d. ii’,
o 1c,IM,rc,tnI y rI,it,j
xFirld ikilo
TI i )
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Lens holography, with a high quality microscope as a lens, can make 1 im
particles visible, but the depth of field and viewing angle suffer greatly.
Using forward—scattered light with a lensless system, investigators have
detected individual submicron particles. 1257 Presumably, the intensity of the
speck of light in the reconstructed image is proportional to the size of the
particle. Work is presently being carried out in this field.
APPLICATIONS TO STACKS
Very recently holography has been applied to coal—fired combustion chamber
environments.1279,j257 The apparatus used, shown in Figure 5, is a 3—beam forward—
scattered light system. The 3—beam system allows holograms to be made even though
the scene beam intensity has been strongly attenuated while passing through the
stack. This system, or one similar to it, successfully detected the cloud density
of the particles in the combustion chamber. The reconstruction apparatus shown
in Figure 6 permitted the reconstructed image to be scanned by a photocell mbich
measured the relative cloud intensity at any point in the viewing volume. The
3—beam technique allowed the absolute intensity information to be carried on the
hologram. Stack vibrations were no problem because the laser pulse time La
very short.
Holography for the analysis of single particles in stacks is more difficult
because of the lack of resolution with the necessary recording distances.
Development remains to be done in this area.
CONCLUSIONS
For use in stacks, the primary application of holography appears to be as
a specialized tool in studying the spatial density of clouds of particles with-
out disturbing the effluent stream in any way. The use of holography for analysis
of individual particles is limited, with present technology, by the lack of
resolution across normal stack distances. Holography will probably remain expensive
and complex for routine monitoring of stack effluents.
The field of holography is developing very rapidly. Statements made here
are based on a brief review of present technology. Developments being made even
as this is written will undoubtedly make large improvements in the applicability
of holography to the measurement of aerosols.
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Fig. 5. Schematic diagram of scattered light three—beam
transmission holocamera test setup. 1257
GLASS WEDGE BEAM
CORNER PRISM
EXPANDING AND
COLLIMATING TELESCOPE
.SCENE BEAM
3 INCH DIA. COLLIMATED’
RE FE RE NCE. BE AM
18” x 18” x 74”
FRONT SURFACE
GLASS WEDGE
BEAM SPLITTER
FRONT SURFACE MIRROR
INTENSITY
HOLOGI
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PHOTODETECTOR REFERENCE INTENSITY SEAM
AND APERIU
INSTRUCTED
AL IMAGE
u .RAII
SCATTERING C IAMBER LENGTH
I 74 119
DISTANCE IN INCHES
Fig. 6. Schematic diagram of method used to reconstruct and
measure hologram real image light intensity.variátions
as a function of scattering chamber lengthJ 257
REFERENCES
682 Belz, R. A., “Analysis of the Techniques for Measuring Particle Size
and Distribution from Fraunhofer Diffraction Patterns”, ARO, Inc.,
Clearinghouse No. AD 674 741 (Sep 1968).
132 DeVelis, J. B., Parrent, G. B., Jr., and Thompson, B. 3., “Image
Reconstruction with Fraunhofer Holograms”, Optical Society of America
Journal , V. 56, p. 423—427 (1966).
1195 Fowles, F. R., Introduction to Modern Optics , Holt, Rinehart, and
Winston, Inc., New York (1968).
1279 Matthews, B.J.., and Kemp, R. F., “Holographic Determination of Injected
Limestone Distribution in Unit lOof the Shawnee Power Plant”, TRW
Systems Group, Redondo Beach, Calif., TRW Report No. l4103—6001—R0—0O
under NAPCA Contract CPA 70—4 (Jun 1970).
1257 Matthews, B. J., and Kemp, K. F., “Investigation of Scattered Light
Holography of Aerosols and Data Reduction Techniques”, TRW Systems
Group, Redondo Beach, Calif., TRW Report No. 14103—6002—RO—00 under
NAPCA Contract CPA 70—4 (Nov 1970).
218 Meier, R. W., “Magnification and Third—Order Abberations in Holography”,
Optical Society of America Journal , V. 55, p. 987—992 (1965).
1196 Anon., “Model 177 Holographic Camera”, Optics Technology, Inc., 901
California Ave., Palo Alto, Calif. .94304
- -.__- --c
RE
L
.10 0
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1197 Reynolds, .G. 0., and DeVelis, 3. B., Theory and Application of
- Holography , Addison—Wesley, Reading, Mass. (1967).
126 Reynolds, ‘C. 0., and DeVelis, 3. B., “Hologram Coherence Effects”,
IEEE Trans. , V. AP—15, p. 41—48 (1967).
148 Silverman, B. A., Thompson, 8.3., and Ward, 3. H., “A Laser Fog
Diadrometer”, Journal of Applied Meteorology , V. 3, p. 792—801 (1964).
1198 Anon., Stat Volt Company, “Holographic Particle Determination
Literature ’, 11’30 Channel Drive, Santa Barbara, Calif.
150 Thompson, B. J., Parrent, C. B., Ward, 3. H., and Juath, B., “A
Readout Technique for the Laser Fog Diadrometer”, Journal of Applied
Meteorology , V. 5, p. 343—347 (1966).
483 Ward, J., “Laser Fog Disdrometer System”, Technical Operations, Inc.,
Burlington, Mass., Clearinghouse No. AD 656 487 (Jun 1967).
253 Zinky, V. R., “Hologram Camera and Reconstruction System for Assessment
of Explosively Generated Aerosols”, Technical Operations Research,
Burlington 1 Mass., Clearinghouse No. AD 474 534 (Oct 1965).
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ACOUSTICAL ATTENUATION AND DISPERSION
by: John Borgos
INTRODUCTION
Acoustical attenuation is the decrease in amplitude of an acoustical
wave traveling through a media due to such effects as vail losses, viscous
losses due to interaction with particles, etc. The coefficient of attenuation,
, is defined for a particulate medium such that the ratio of the sound intensity
at a distance £ from the source to the intensity at the source is given by
exp(—u U. Acoustical dispersion is related to the decrease in sound velocity
due to the presence of partiâulate matter in the conducting medium. The dispersion
coefficient, B, is defined as
B (a ,/a) 2 — 1,
where a and a 0 are the sonic velocities in the gas with and without particles, respec-
tively ’ 03 .
There are several attractive characteristics of the use of acoustic measure-
ments of aerosol concentration which other measurement techniques can seldom
duplicate. However, the acoustic method require. a knowledge of particulate density
and the gas properties. One characteristic of the acoustical. method which makes
it appear much less promising is that it is not nearly a. sensitive a. other
techniques to aerosol mass concentration. This is a serious limitation.
The most comprehensive work on the subject of acoustical measurements of
aerosol size and concentration has been done by Tenkin and Dobbins’ 03 , 138 ,l 39 .
They have outlined a theory which explains the phenonema of acoustical attenuation
and dispersion using the mechanisms of the thermal and dynamic relaxation processes,
and they have produced considerable experimental data to verify their conclusion.
Much of their work is based on previous work, the earliest notable being that of
Knudson et al 605 , who made measurements of the rate of decay of sound in a medium
with and without particles present. Later, Epstein and Carhart 616 included the
effects of thermal attenuation of their theory, which agreed fairly well with
Knudson’s data. Zink and Delsaeso 604 also reported a theory that agreed quite
well with their experimental results. The work of Tenkin and Dobbina, while similar
to much of the previous work, presented more accurate results over a wide range of
conditions.
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AN Y IS OF BASIC PRINCIPLES
When a pure ‘gas is exposed to acoustic waves, the waves experience
practically no attenuation, except that caused by the presence of discontinuities
such as confining walls. If the same gas contains an aerosol of sufficient con-
centration, an acoustic wave will experience measureable attenuation while
propagating through the gas, and its propagation velocity will be less than it
was in the particle—free gas. The attenuation is measured in terms of the
attenuation coefficient, a, and the change in propagation velocity is measured
in terms of the dispersion coefficient, 8.
Consider a particle immersed in a gas that is isolated from any other
particle in the gas, If an acoustic field is applied, the motion of the particle
will be affected ‘by the motion of the gas molecules as the sound wave propagates
through the medium. If the particle is extremely large, it will remain nearly
motionless becau e of its inertia. As the particle size decreases, the particle
tends to oscillat e more closely with the sound field. This follows from the fact
that the inertia, which resists particle motion, changes as B 3 ; on the other hand,
surface area, which is proportional to the viscous drag forces, only changes as
B 2 . Thus, for an extremely small particle, the amplitude of the particle oscillat-
ing motion approximates that of the gas molecules. For a range of intermediate
size particles, the particle motion is somewhat less than the motion of the gas.
Bobbins and Temkin ’ 38 have verified experimentally that this is indeed what
happens in the case of a plane wave and nearly spherical particles. In fact, the
intermediate particles not only vibrate with smaller amplitudes, but they lag
behind the motionof the gas molecules. The important parameters in the deter-
mination of part itle lag for a particular particle size are uYrd and wr , where w
is the circular frequency of the sound field and T d and it are the dynamic and
thermal relaxation times, respectively, of the particle in the medium.
Two mechanisms account for practically all of the acoustic attenuation caused
by particles. The most important is the viscous attenuation caused by the lagging
motion of the pai?ticles, which causes viscous energy loss due to the relative
motion of the particle and gas. The dynamic relaxation time, t, 1 , is a measure of
this contribution to the attenuation. The other mechanism is the thermal
attenuation, which is a result of the irreversible flow of heat between the suspended
particle and the gas during the rarefactions and compressions of the sound wave.
Though usually smaller than the viscous contribution, 139 it is usually of the same
order of magnitud’é. The thermal relaxation time, ‘re, provides a measure of the
thermal attenuation for a given frequency and particle size.
The dispersiià, or velocity change, of a sound wave due to the presence
of particles in agas is simply the result of the combined effects of the in-
crease in heat capacity and increase in density of the gas particle mixture.
The increase in h%at capacities resuj.ts in a lower ratio of specific heats.
Because the acouètic velocity is projrnrtionai to the square root of the specific
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heat ratio and inversely proportional to the square root of density, the
presence of particles in a gas can only decrease the sound velocity in the 138 139
niedium. 604 The dispersion can also be shown to be a function of and ° d’
The equipment used by Dobbins and Teinkin 138 ’ 139 in obtaining experimental
data, usually referred to as an acoustical interferometer, consisted of a hollow
cylinder, one end of which was plugged by a rigid cap, while the other end con-.
tamed an acoustical driver. The acoustical driver was used to set up a standing
wave pattern in the tube, and the resulting preasut e field was explored with a
capillary tube attached to a microphone. The dispersion of a sound wave due to
the presence of particles was found by measuring the change in wave length of
a sound wave of a particular frequency when particles were added. The attenuation
was determined by measuring the decrease in amplitude of the fluctuating pressure
when particles were added.
The aerosol mass concentrations used in the experiments were on the order of
one part per hundred. Interaction with the walls of the tube can be neglected at
such high concentrations, so the attenuation measured was due almost entirely to
the particles in the tube. Acoustic frequencies varied from 1 KHz to about 10 KHz,
and the mean particle size varied from 0.8 to 4.7p.
Data shows that for Wtd 1, maximum attenuation occurs. If either the
frequency or mean particle size increases or decreases, the attenuation drops
sharply. The magnitude of the maximum attenuation is weakly dependent on the
particulate mass concentration for Cm > 0.01, where Cm is the ratio of the total
mass of particles to the mass of gas in a given volume of the mixture. For C, < 0.01,
the maximum attenuation is not dependent on Cm (Fig. 1). The data also shows that
as wrd - 0, a maximum dispersion occurs. This maximum is also dependent on mass
concentration for Cm > 0.01, and independent of mass concentration for Cm < 0.01.
As WTd - , the dispersion tends to zero (Fig. 2).
Fig. 1: Attenuation by Water Droplets Fig. 2: Dispersion by Water Droplets
in Air.
C L
WI 4
In Air
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The measurement of a mean particle diameter appears possible if a measure-
ment of mass concentration is obtained separately. This mean diameter is the
same diameter that is measured by an optical—scattering photometer; namely,
the volume—surface mean diameter. 138
The measurements of mean diameter and mass concentration of particles has
been shown theoretically to be nearly independent of the particle size distribution
function, ’ 38 and although the range of the data is limited, it seems to confirm
this.
APPLICATION TO MASS CONCENTRATION MEASUREMENT IN STACKS
There are several problems connected with using an acoustic interferometer
for monitoring particulate mass concentration in stacks that do not appear to have
immediate solutions. The outstanding problem is the lack of sensitivity. The mass
concentration of particles used in most of the experiments thus far has been on
the order of one part per hundred, which is about three orders of magnitude greater
than the lower limit of concentration encountered in stacks. Furthermore, even at
such high mass concentrations, the instrument is still insensitive to higher con-
centration. Even in a stack, the instrument would need to measure a sound velocity
change of 0.01% in order to detect any particles at all, and would have to measure
velocity within 10—4% to accurately measure a change in concehtration. Both
appear unfeasible.
In addition to this, there are problems involved in the use of the sensing
tube in a stack. Contamination of the acoustic driver, the need for a discontinuous
isokinetic sampling system (the air should be at rest in the sensing tube when
measurements are made, so the sampling system would have to be able to fill the tube,
empty it, fill it, etc.), settling of particles in the sampling tube, and the effect
of temperature all need to be carefully examined.
CONCLUSIONS
Because of the extreme sensitivity requirement, which is impossible to attain
even in a laboratory environment, the acoustic technique cannot be considered for
mass concentration measurements in stacks. If a satisfactory technique is developed
which makes it possible to increase the particle mass concentration by three orders
of magnitude in a given sample, it may be feasible to make mea urements in the
concentrated samples which give meaningful data. Otherwise no apparent use can be
made of the technique. -
RECOMMENDATIONS
Since the overwhelming problem with the acoustic technique lies in the
required sensitivity for the normal range of mass concentration values, an
instrument is needed which will concentrate the particles. Such an instrument
might be a version of a cyclone, designed such that the particles were not
separated from the air but remained in suspension.
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REFERENCES
943 Anon, “Applications of Ultrasonic Energy, Ultrasonic Instrumentation
for Nuclear Applications”, Aeroprojects, Inc., West Chester, Pa,
Clearinghouse No. NYO—9585 (Oct 1961).
600 Blair, D. W., “Particle Damping of a Plane Acoustic Wave in Solid
Propellant Combustion Gases’ t , AIM Journal , V. 1, p. 2625—2626 (1963).
314 Dobbins, R. A., and Temkin, S., “Measurements of Particulate Acoustic
Attenuation”, AIM Journal , V. 2, no. 6, p. 1106—1111 (Jun 1964).
103 Dobbins, R. A., and Tetakin, S., “Acoustical Measurements of Aerosol
Particle Size and Concentration”, Journal of Colloid & Interface Science ,
V. 25, no. 3, p. 329—333 (Nov 1967).
616 Epstein, P. S., and Carhart, R. R., “The Absorption of Sound in
Suspensions and Emulsions”, Journal of the Acoustical Society of America ,
V. 25, p. 553—565 (1953).
557 Gucker, F. T., Jr., “Determination of Concentration and Size of Partic-
ulate Matter by Light Scattering and Sonic Techniques”, Proc. National
Air Pollution Symposium , 1949, V. 1,. 14—26 (1951).
605 Knudson, V. 0., Wilson, J. V., and Anderson, N. S., “The Attenuation of
Audible Sound in Fog and Smoke”, Journal of the Acoustical Society of Americ
V. 20, p. 849—857 (1948).
139 Teuakin, S., and Dobbins, R. A., “Attenuation and Dispersion of Sound
by Particulate — Relaxation Processes”, Journal of the Acoustical
Society of America , V. 40, no. 2, p. 317 334 (Aug 1966).
138 Temkin, S., and Dobbins, R. A., “Measurements of Attenuation and
Dispersion of Sound by an Aerosol”, Journal of the Acoustical Society
of America , V. 40, no. 5, p. 1016—29 (Nov 1966).
604 Zink, J. W., and Delsasso, L. P., “Attenuation and Dispersion of Sound
by Solid Particles Suspended in a Gas”, Journal of the Acoustial Society
of America , V. 30, p. 765—771 (1958).
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HOT-WIRE ANEMOMETRY
by: John Borgos
INTRODUCTION
A hot—wire anemometer placed in an airstream with liquid aerosol will
display voltage output fluctuations due mainly to twO phenomena: the transient
cooling effect due to the turbulence level in the airstream and the impaction
on the sensor of the li4uid droplets. If the thermodynamic properties of the
liquid and airstream are known, the two effects can be discriminated, and measure-
ments can be made of each of the two by a single system.i°
As a monitor of mass concentration, the hot wire anemometer has several
advantages which are important in a large number of applications. The main
advantage is that it does not disturb the flow significantly in most cases.
This is especially important when concentration at a point must be measured,
An extraction probe which removes a sample is often the only lubstitute, a method
which usually results in some particle loss in the probe. Also, if the flow is
turbulent and the mean velocity is small, an extraction probe may not be ideal
because the local velocity may change direction too frequerit ].y for the probe to
follow.
Most of the experimental work done thus far with a hot wire anemometer has
been done by Goldschmidt. 10 ’ 14 His hot wire was quite small (4.5 m diameter).
Most of the droplets he measured had diameters largerthan the wire. Most of
his experiments were on dibutylphthalate droplets, although he also experimented
with water, Sinco—Prime 70, aüd safflower oil.
PRINCIPLES OF OPERATION
A hot wire anemometer can be operated either by maintaining constant current
or constant temperature in the wire. In either case, a local velocity fluctuation
or deposition of a drpplet on the wire will result in a transient voltage change
across the wire, These voltage fluctuations can be discriminated through a set of
electronic filters, and the particles can be counted by sending the filtered signals
through a high—frequency electronic counter.
With the experimental apparatus used by Goldschmidt, 1 ° the droplet sizes
which could be sensed ranged from a minimum of roughtly 3 im to approximately 200 Im,
the minimum being fixed by the noise level of the system, and the aerosol and wire
properties. Since the wire diameter was 4.5 tm, the mechanisms of inertial Impaction
and interception were both important in causing particles to hit the wire. 79 Thus,
Goldschmicit determined the droplet collection efficiency of. the wire experimentally
to avoid errors associated with various theories.
TH ERMO- SYSTEMS iNc.
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—221—
He also assumed that a droplet which came in contact with the wire would
wet the wire and quickly vaporize. Photographs later showed that this was
almost always the case, with the exception of collisions where the particle
only touched the side of the wire. In these cases, the droplet did not seem
to vaporize completely, thus producing a smaller cooling effect than a droplet
intersecting the wire near the stagnation point.
As stated, the impingement of a droplet on the wire causes a voltage
fluctuation. This is because the droplet is cooler than the wire and requires
a large amount of heat to vaporize. A quantitative measurement can be made of
this heat by observing the magnitude of the voltage fluctuation and correlating
it with power output. The magnitude of the voltage fluctuation is primarily a
function of the temperature difference between the wire and the droplet, the size
of the droplet, and the specific heat of the droplet, with surface tension of
the droplet and wire tension producing lesser effects.
If the hot wire is exposed to a droplet—laden airstream, the output can be
filtered (to eliminate turbulence — generated fluctuations) and examined on an
oscilloscope. With a knowledge of the droplet’s specific heat and temperature
and of the wire temperature, the s rs tern can be calibrated to measure particle
size. Goldschmidt and Householder ° have shown that for droplets smaller than
200 pm in airatreams of moderate velocity (Vc300 cm/sec), the peak value of the
voltage fluctuation is linearly proportional to particle diameter. Droplets
smaller than about 3 am are difficult to detect because of the noise level of
the electronics.
CONCLUSIONS
The hot wire anemometer is not applicable to the measurement of particulate
mass concentration in stacks. Its main disadvantage is its inability to measure
solid particles. A solid particle does not vaporize on contact with the wire,
but instead coats the wire and actually decreases the heat transfer. Thus, solid
and liquid particles yield completely different output signals. The signal caused
by solid particles cannot easily be interpreted, making the hot—wire anemometer a
poor candidate for stack monitoring of particle concentration or size distribution.
REFERENCES
10 Goldschmidt, V. V., and Householder, M. K., “The Hot Wire Anemometer
as an Aerosol Droplet Size Sampler”, Atmospheric Environment , V. 3,
p. 643—651 (1969).
14 Goldschmidt, V. V., “Measurement of Aerosol Concentrations with a Hot
Wire Anemometer”, Journal of Colloid Science , V. 20, p. 617—634 (1965).
79 Golovin, M. N., and Putnam, A. A., “Inertial Impaction on Single Elements”,
Indust. & Eng. Chem. Fundamentals , V. 1, no. 4, p. 264—273 (Nov 1962).
224 Vonnegut, B., “Means for Measuring Individual Aerosol Particles”, U. S.
Patent No. 2,702, 471 (Feb 1955).
. . . a a flt,a.. at ..ir
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—222—
PRESSURE DROP IN NOZZLES
by: John Borgos
INTRODUCTION
The use of nozzles and venturi meters as instruments for measuring
particulate mass flow in particle-laden gas streams has been confined mainly
to applications where the transportation of the particles was of primary
interest. The technique has not received much recognition in the pollution
control field because it is suited for the monitoring of particle concentrations
that are much higher than those normally found even in heavily polluted air.
ANALYSES OF BASIC PRINCIPLES
If a gas stream if passed through a venturi nozzie, the flow rate can be
calculated using the well—known relationships between pressure drop and velocity.
The presence of particles affects these relationships by a mechanism that is
not defined quantitatively. 103 ’ The principle involved is that the particles
do not instantly follow the velocity change of the gas in the constriction of the
nozzle, but lag behind. In the diffusor section, on the other hand, the particles
decelerate slower than the carrier gas.
Barth and Weber 225 have designed a venturi such that the pressure differential
between the throat and the diffusor inlet Is linearly related to the dust quantity
passing the nozzle per unit time. The relationship is dependent on the size
distribution of the particles, which limits its use to applications where the size
distribution is constant.
RECENT APPLICATIONS
The instrument designed by Barth and Weber has been used to measure the dust
content of industrial emission sources. Because its design limits it to specific
dust contents of 500 gIm 3 or more, it had to be modified to accommodate normal
emissions dust contents of about 500 mglm 3 . This was done by a cyclone used as
an amplifier to enrich the concentration. The experiment was not entirely sat-
isfactory because the periodic nature of the separation mechanism of the cyclone
caused severe pressure fluctuations 225 .
A. nozzle has been used in conjunction with an orifice plate to successfully
measure both the velocit 1 of carrier gas and mass flux of particles in a stream
laden with coal dust. 1 ° 3 The orifice is insensitive to the presence of particles,
so a measure of velocity can be obtained by its pressure drop. With this data, the
pressure drop across the nozzle can be used to measure particle concentration.
Errors as low as ± 5% have been claimed or coal—air ratios between 0.5 to 1.4 lb.
coal per lb. of air.
THERMO• SYSTEMS INC.
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—223—
CONCLUS IONS
The pressure drop method, as has been stated, cannot be directly applied to
airstreatna whose dust content is below approximately 500 g/m 3 . Since no sat-
isfactory method is available that can be used to increase concentration of normal
stack gases to that level, the pressure drop method cannot be made sensitive
enough to be useful for mass concentration measurements in stacks.
RECOMMENDATIONS
The decisive problem that inhibits further development of this method lies
in its inability to detect particle mass concentrations as low as those found in
stack gases. Therefore, the discovery of a highly efficient separator that operates
continuously and provides a particle—enriched airstream would constitute a major
breakthrough.
REFERENCES
1067 Barth, W., Nagel, P.., and Waveren, K., “New Method for the Instantaneous
Determination of Material Quantities Moved in Air Stream of Pneumatic
Conveyers”, Chemie—Ing. Techn , V. 29, no. 9, p. 599—602 (1957).
1031 Beck, M. S., and Wainwright, N., “Current Industrial Methods of Solids
Flow Detection and Measurement”, Powder Technology , V. 2, p. 189—197
(1968).
225 Duwel, L., “Latest State of Development of Control Instruments for the
Continuous Monitoring of Dust Emissions”, Staub—Reinhalt der Luft
( Engi. Trans.) , V. 28, no. 3, p. 42—53 (Mar 1968).
1029 Gast, T. H., “Determination of Solids in Hot Waste Gases Using Electrical
Micro—Balance”, Chemie—Ing. Techn. , V. 29, p. 262—266 (Apr 1957).
•1 11 &Is • #‘
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«jrar«> of the Art- 1971 Instrumentation for Measurement of
Peculate Emissions Frfcm Combustion Sources Volume II:
Particulate Mass-Detail Report
_____
9. Performing Organization Name and Address
Thermo-Sy stems Inc. ,
2500 North Cleveland Avenue
St. Paul, Minnesota 55113
3. Recipient's Accession No.
. Report Date \
April 1971 . \
6.
}• Performing Organization Kept
No.
10. Project/Task/Work Unit No.
I). Contract/Grant No.
' * 'f
CPA. 70-23
11 Sponsoring Organization Name and Address
Environmental Protection Agency
Air Pollution Control Office
Division of Process Control Engineering
Research Triangle Park, North Carolina 27711
13. Type of Report & Period
Covered
14.
- This report was furnished to the Office of Air Progri
North Cleveland Avenue, St. Paul, Minnesota 55113 in
fulfillment of Contract No. CPA 70-23
u
, ;A11 known sensing techniques avaiiaoie tor application co automatic, con-
rnoaaurement of the rate of particulate mass emissions from large fossil-fuel
S£ fSEilities are discussed. The measurement of particle mass, rather than
rticle parameters is emphasized. Although the report emphasizes permanently-
' effluent monitoring systems, much of the information is also applicable to
ind research instruments. Detailed discussions* of particle "sensing techniques
Sd to missions monitoring are presented. Each discussion analyzes 'possible
problems and their solutions, in using the technique for emission monitoring, and in-
cludes an analysis of what particulate parameter the technique sees, how closely the
measurement correlates with particulate mass, inherent measurement errors-, practical
measurement cw"c *.«»«•«= or - ... „ «.»,., -nt.ant-ia-\ aonalMvit-v and reammae of eai
.
^
senarate chapter summarizes many. of the problems encountered in the. design ot sampling
probes required by most of the particle sensing techniques.^
17. Key Words and Document Analysis. 17a. Descriptors
Air pollution
Measuring instruments
Particles
Exhaust emissions
Combustion products
Continuous sampling
Flue gases
17b. Identifiers/Open-Ended Terms
»
17c. COSATI Field/Group 13B
19. Security Class (This
Report)
•~":LAsaiFi
21. No. of Pages
IB. Availability Statement
Unlimited
20. Security Class (This
UNCLASSIFIED
22. Price
USCOMM-DC 4032B-P71
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